crops by nasif123

VIEWS: 58 PAGES: 30

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        Applied Botany – Arable Farming
                 Cereal Crops
                                                 []


Agricultural production has increased enormously in the last few decades, and seems set to rise
even further. These increases have been brought about mainly by the development of new
varieties of crops, animals and more intense methods of farming.


Maize                                                                                              []
                       Originating in Central America, maize is now third only to wheat and rice in
                       world production. It is widely grown in the USA (and Europe) as animal
                       feed, and is also the basis of Corn Flakes and Sweetcorn for human
                       consumption. This crop grows well where temperature is frost-free and
                       light intensity is high. There needs to be adequate water too – though
                       not as much as rice needs.

                       It is grown as a staple food in much of Africa. However, for this purpose it
                       has a major drawback, since it is deficient in the essential amino-acids
                       tryptophan and lysine. This causes children after weaning (i.e. about 4-7
                       years old) to become ill. Their livers greatly enlarge in an attempt to
                       synthesise the missing amino-acids and they suffer from Kwashiorkor.
The symptoms include stick-like arms and legs, thin, papery skin and a greatly swollen belly.
Note that these children are not ‘starving’ – they may have plenty of calories in their diet – but
they are malnourished.

Geese (and, less commonly, ducks) fed largely on a diet of maize develop similar symptoms.
Their grossly enlarged, fatty livers are then used to make paté de fois gras – mainly in the
Bordeaux region of France. This is a rare example of a farmer deliberately making an animal
seriously ill in order to maximise profit – the reason why I will not eat it! Another example is veal
where calves are made anaemic, by depriving them of grass, in order to make their meat paler.

 One might assume that if maize crops are grown in conditions where light intensity and daytime
temperatures are high; then the conditions should favour photosynthesis, however this is not
necessarily the case as:

    •   high temperatures increase the rate of transpiration, leading to the closure of the
        stomata. Closing the stomata can cause a build up of oxygen from photosynthesis in the
        leaves – this can reduce the photosynthetic yield.
    •   if plants are grown close together, then there will be competition for carbon dioxide.


Adaptations include:

    •   A different biochemical pathway for photosynthesis (with an extra step) than that in most
        cooler climate plants. Called the C4 pathway, these plants can fix carbon dioxide at low
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    levels as the four-carbon molecule malate. This molecule is then used to boost CO2 in
    the regular C3 pathway in a different cell. This mechanism allows photosynthesis to
    continue at higher rates, since the oxygen produced in the light reactions (see Module 5!)
    is no longer inhibiting the process. The normal limiting factor in the UK for photosynthesis
    is low CO2 – at our current CO2 levels of around 370 ppm (and rising!). Normal C3 plants
    are inefficient and fail to grow at concentrations below about 200 ppm, whilst C4 plants
    can successfully ‘fix’ CO2 at levels as low as 0.1 ppm. At 370 ppm, they grow faster.
•   Remember – oxygen competitively inhibits the key carbon – fixing enzyme in the light-
    independent reactions of photosynthesis (see module 5!), called RUBISCO. i.e. beyond
    ‘dim’, the brighter the light, the slower photosynthesis eventually becomes.
•   The roots are shallow but widespread, so maize often has small aerial roots at the base
    of the stem to increase their ability to withstand buffeting by wind (called buttress
    roots).




                                                (Provided by: Illinois World Food and Sustainable Agriculture
                                                                           Program)




                                               Sorghum                                                      []
                                               Is the fifth commonly grown cereal in the world
                                               and is another tropical C4 cereal, like Maize
                                               (see above). Sorghum is adapted to hot, arid,
                                               low-soil nutrient conditions, reflecting its
                                               origins in the Sudan region of Africa. In the
                                               drier regions of Africa and Central India it is
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often a staple food, being made into a tasteless porridge, but in the rest of the world it is used as
animal feed or as a source of oil and fibre. The USA is the major grower of Sorghum for this
purpose. In the UK we come across it as millet – used as budgie food!

                                                                     o
Sorghum is able to grow well in the very hot (over 35 C), dry regions of tropical Africa, southern
USA and central India. It is able to do this by synthesising special ‘heat-shock’ proteins very
rapidly when the temperature rises. It grows very high – up to 5 metres in a season – and the
multiple seed-heads produce many thousands of small seeds from a single plant.

Xerophytic (= a plant normally found in dry conditions) adaptations include:

    •    A dense root system that is very efficient at extracting water from the soil (both wide and
                                                              deep).
         •        A thick waxy cuticle that prevents evaporative water loss through the leaf surface.
     •   The presence of special cells (called motor cells) on the underside of the leaf that cause
         the leaf to roll inwards in dry conditions. This traps moist air in the rolled leaf and reduces
                                                            water loss.
                     •    Reduced number of sunken stomata on leaves (they are in pits).




                                   World sorghum growing areas
                               (provided by: syngenta foundation for sustainable agriculture)




Rice         []


                                                  The second most widely grown cereal (wheat is top!),
                                                  rice is grown throughout the tropical and Mediterranean
                                                  regions of the world. It requires a minimum temperature
                                                  of 20oC in the growing season. It is the staple food of
                                                  half the World’s population, being highly nutritious and
                                                  needs little post-harvest processing to make it edible.
                                                  Despite its Asian origins, it is a C3 plant; the USA is the
                                                  world’s biggest exporter, since most poor countries can
                                                  barely feed their own populations. Of the two main types
                                                  of rice (mountain and plains rice) only the latter has
                                                  any adaptations to an unusual habitat - uniquely, it can
                                                  grow in flooded conditions (though it is not a true
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hydrophyte or water plant). This is the variety grown in SE Asia, where it is grown partly
submerged in paddy fields for some of its life. These flooded soils ‘drown’ weeds, reducing
competition.

The land is not needed for the seedling stage of growth either, so enabling up to 3 crops per
year. It is, however, expensive in labour – though this is the traditional way of life for women
and children in this part of the world.

Adaptations of plains rice include:




                                Rice stem showing its hollow and
                                       aerenchyma tissue

    •   The stem of a rice plant has large air spaces (hollow aerenchyma) running the length of
             the stem and into the roots. This allows oxygen (some formed in the plant from
            photosynthesis) to penetrate through to the roots which are submerged in water.
    •   The roots are also very shallow, allowing access to oxygen        that   diffuses   into the
        surface layer of the waterlogged soil.


    •   When oxygen levels fall too low, the SEEDLING (only!) cells can respire
        anaerobically, producing ethanol. Ethanol is normally toxic to cells, but the young root
        cells of rice have an unusually high tolerance to it – they have large levels of the enzyme
        alcohol dehydrogenase in their cells. Adult plant roots are as intolerant of flooding as
        any other crop.    Note that this is a physiological adaptation, whilst all the others
        mentioned in this section are physical or anatomical adaptations.
    •   When germinating, the seed grows rapidly, forcing a hollow tube or coleoptile upwards.
        This eventually breaks through the surface of the water, forming a ‘snorkel’ - through
        which the leaves eventually grow – allowing oxygen to penetrate to all parts of the plant.
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Paddy fields are ‘bogs’ made by flooding the field with river water and also the local sewage
(both animal and human). This makes the water rich in organic matter (if rather smelly!).
Microbes break down the sewage whilst others use this energy to ‘fix’ nitrogen from the air. Fish
may also live there, feeding on the animal life. When the crop is about to flower, the field is
allowed to drain naturally and the bacteria break down in the soil releasing nitrogen for the benefit
of the rice crop (catching the fish would be easy too!). Thus a paddy field is both sewage and fish
farm and a fertilizer factory. Rice yield drops dramatically when weeds are present, so this ‘anti
weed’ system is important.




                      (Provided by: Illinois World Food and Sustainable Agriculture Program)




Wheat         []
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 This is the world’s most widely-grown crop and is extensively grown throughout the temperate
regions of the world, both as human (flour) and animal feed. Bread wheat – durum wheat - is a
hard wheat, with a high protein (gluten) content, which enables the dough to stretch when rising.
It is also excellent for making pasta! Spring-sown, it is the preferred variety of Eastern Europe,
Canada and the mid-west USA. Winter wheat is a soft wheat, with a low gluten content and is
good for making cakes and biscuits. It is grown throughout the UK and Western Europe and in
more temperate climates as it has a higher potential yield. It is also ideal for animal feed, since it
is easier to digest.




                       (Provided by: Illinois World Food and Sustainable Agriculture Program)

           Adaptations and cropping yields of the major cereal crops


                                      Yield                       Area grown
         Crop                              -1                              6               Growth requirements
                                    (kg/ha )                       (ha x 10 )
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                                                                         Warm, frost free
        Wheat                    1000 - 4000               215           climate, fertile soil,
                                                                         drought intolerant
                                                                         Adapted to a wide
        Maize                1000 - 14500                  139           range of temperate
                                                                         climates and soils

                           1500 (mountain)                               Tropical, paddy
         Rice                                              155           varieties are aquatic,
                                 4500 (plains)
                                                                         drought intolerant

                                                                         Wide range of soils.
                                 300 – 2000
                                                                         Drought tolerant.
      Sorghum                                              43
                           [6500 if irrigated]                           Grown in regions too
                                                                         dry for maize.


                          Food values of some major crops


                            Energy (kJ/gm)            Protein (%)        Lipid (%)
                Crop

                Wheat                1420                12.1               2.1

                Maize                1471                10.1               4.1

                Rice                 1296                 8.1               2.1

            Sorghum                  1455                10.1               5.1

            Potatoes                  347                 2.0               0.1

                Peas                  293                 4.9               0.4

             Lettuce                   63                 1.2               0.2




                         Farming Methods
                                                 []



Intensive Farming           []



After World War II intensive farming techniques flourished. Using high-yielding hybrid cultivars
and large inputs of inorganic fertilisers; newly-developed chemical pesticides, and machine
power, crop yields increased to 3 or 4 times those produced using the more extensive (low-input)
methods of 100 years ago. Large areas planted with monocultures (single crops) are typical.
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Irrigation and fertiliser programmes are often extensive to allow the planting of several crops per
season. Given adequate irrigation and continued fertiliser inputs, yields per hectare from intensive
farming are high. Over time, these yields decline as soils are eroded or cannot recover from
repeated cropping. More fertiliser leaches from the soil and enters groundwater as a pollutant.
Unemployment rises as workers are replaced by machines and the farmer becomes more and
more dependent on fossil fuels and external inputs – he becomes a ‘land-slave’. On the other
hand, famine has become a thing of the past.



Extensive (or Organic) Farming                    []


     Organic farming is a sustainable form of agriculture based on the avoidance of synthetic
  chemicals and applied inorganic fertilisers. It relies on mixed (crop and livestock) farming and
 crop management, sometimes combined with the use of environmentally friendly pest controls
 (e.g. biological controls and flaming), natural pesticides (nicotine and derris) and livestock and
  green manures. Note that ‘Organic’ is not the same as ‘pesticide-free’; to have that you must
 grow your own - and eat the occasional pest yourself! Organic farming uses crop rotation and
  intercropping, in which two or more crops are grown at the same time on the same plot, often
 maturing at different times. If well cultivated, these plots can provide food, fuel, and natural pest
 control and fertilisers on a sustainable basis. Yields are typically lower than on intensive farms,
   but the produce can fetch high prices, and pest control and fertiliser costs are reduced. It is
                         labour-intensive, but requires little external input.




Comparison of agricultural methods

               Intensive farming                             Extensive (organic) farming

                  Advantages                                          Advantages
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                                                 •   More closely matches crop type to
•   World grain production has almost                appropriate season and soil.
    tripled in the last 50 years.                •   Increases crop diversity and breaks up
•   Per capita grain production has                  disease and pest cycles.
    increased, reducing global hunger.           •   Soil quality and structure improve,
•   The cost of food has declined: more              reducing nutrient & water loss.
    food is now traded globally.                 •   Farmers can still make use of new high
•   Yields increase more quickly and                 yielding varieties of crops.
    effectively than with the alternatives.      •   Produce is pesticide-free and produced
                                                     in a sustainable way.




            Disadvantages                                   Disadvantages




                                                 •   Yields are lower and more land is
                                                     required for the same output.
•   Increases in yields may not be               •   Organic produce may have more
    sustainable: the rate of gain in total           blemishes and a shorter shelf-life than
    production is slowing and per capita             sprayed produce.
    production is now declining.
                                                 •   Price may increase to offset losses.
•   Pesticide use is escalating yet pesticide
                                                 •   High use of manures leads to bacterial
    effectiveness is decreasing.
                                                     contamination of produce.
•   Fertiliser use is increasing: soil & water
                                                 •   The choice (especially of out-of-season
    quality continue to decline.
                                                     crops) may be restricted.
•   Poor countries are reliant on outside
                                                 •   use of ‘organic’ or old-fashioned
    assistance and cannot afford the cost
                                                     pesticides may cause more damage to
    of the fertiliser required to achieve high
                                                     the environment than modern
    crop yields
                                                     alternatives
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A fully-equipped modern greenhouse with all growth factors fully controlled by computer
                     - shame about the flavour of the tomatoes!
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Manipulating the growing Environment                                                []



In the open field, Man can only manipulate the growing conditions to a certain extent. The biotic
component of the environment is under control – he can sow at the optimum density, weed and
spray against pests and diseases. But the abiotic component is much les amenable to change;
there is only so much that we can do. We cannot alter the climate (though, through Global
Warming, we may be about to do so), so the sunshine, average temperature, days between first
and last frosts are largely fixed. True, we can plough (to aerate the soil); drain or irrigate (to seek
optimum water levels); plant shelter-belts (to reduce wind speed); lime (to raise the pH – there is
little we can do to lower it); and, of course, add fertilisers (to correct nutrient deficiencies).


But the three limiting factors for photosynthesis – light, temperature and carbon dioxide can only
  be controlled in a greenhouse or laboratory. It is important to realise that the Laws of ‘Limiting
  Factors’ and of ‘Diminishing Returns’ apply here; unless increased profits more than cover the
                               cost, there is no point in altering anything!




Carbon Dioxide            []



Photosynthetic organisms (Prokaryotes, Protoctista (= algae) and Plants) evolved when the
concentration of carbon dioxide (CO2) in the atmosphere was very different from today.
Originally, the Earth’s atmosphere would have held up to 20% CO2 and, when the dinosaurs
became extinct (65 million years ago), the level was still over 5% and oxygen levels would have
been too low for matches to burn, for instance!


In more recent times, studies of the bubbles trapped in ice cores from Greenland and Antarctica
confirm that CO2 levels were lower than today over much of the recent past – e.g. 280 ppm at the
start of the Industrial Revolution (1750) – compared to the 375ppm of today (growing at 1.5 ppm
each year). These fluctuations were associated with climate change – but which was cause and
which was effect is anyone’s guess!


In a greenhouse in full crop, the level of CO2 inside on a sunny day will be well below that outside
and so photosynthesis is slowed considerably. Growers can add extra CO2 by one of two main
methods:
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    1.        burning fossil fuels and releasing the flue gases into the greenhouse (thus raising
         temperature as well – so useful in winter/spring
    2.        adding pure CO2 from a tank of liquid (or ‘dry ice’ = solid CO2) outside. This is very
         cold, so is more useful in summer.


Either method costs money and the levels of CO2 need to be monitored carefully. It is pointless
to raise the levels over 1000 – 1200 ppm, since photosynthesis does not increase, no matter
what the conditions and there is no point at all if the temperature and light are below optimum (as
is likely in winter).




                                               Light   []

    Light is obviously the key ingredient in photosynthesis – as the source of energy, without it
   photosynthesis (= ‘light building’) could not exist. But there is more to light than day or night!


         1.        Light intensity – our eyes and brain are very good at optimising our vision so
         that we do not realise just how gloomy it is inside. Light intensity is measured in ‘lux’ and
         offices normally have around 5-600 lux at desk-top height. In contrast, a sunny summer’s
         day can have well over 40,000 lux – and much more in the tropics! In winter, lack of light
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seriously restricts growth and so growers try to site their glasshouses on south-facing
slopes overlooking the sea (or a large lake).


                                                      Every plant has a light level where
                                                      photosynthesis = respiration and no
                                                      net gas exchange takes place. This is
                                                      known as the Compensation Point
                                                      and is passed every day at dawn and
                                                      dusk. Some plants (‘sun plants’) have
                                                      a high light value (e.g. grasses);
                                                      others (‘shade plants’) have a much
                                                      lower value (e.g. ivy). Trees can even
                                                      have two types of leaf on the same
plant! The anatomy of the two types of leaf is very different – shade leaves are larger,
with two or more palisade layers in the leaf and, consequently, more chlorophyll.
Research has shown that the yield of tomatoes etc. always rises with more light –
gardening books recommending shading your greenhouse in summer are simply wrong!


2.      Colour       – white light is a mixture of colours and

plants cannot use them all. In fact, they need roughly equal
amounts of red and blue light, whilst they reflect green and so
cannot use it at all. [This is dealt with in Module 5!]. Artificial
lighting can be used in winter to boost natural daylight levels –
but it costs a lot!      Filament lamps give too much red,
fluorescent tubes are OK, but give out too much heat, whilst
metal halide and sodium lamps (as used in B&Q etc and in
street lights) are fine – only about 35% of the electricity is
turned to light, the rest wasted as heat. Many commercial
plants are raised in ‘growth rooms’ where the light is all
artificial. Sown in mid-winter, the plants get all the light they need for the first 2 months or
so, before being moved to the glasshouses in February, prior to cropping in May.


3.      Day-length – plants grow best when the days are about 16 – 18 hours long,
which is what we have in summer in the UK. In winter, with 8 hours or less of daylight,
plants scarcely grow at all – even when kept warm. Surprisingly, plants generally do not
like continuous light (they like to ‘sleep’ too!). The length of day has a dramatic effect on
the flowering of most plants. Some like short nights and long days (runner beans); others
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        like short days and long nights (Xmas cactus); by manipulating day-length (by the use of
        blinds as well as artificial lights) it is possible to have pot-grown chrysanthemums all the
        year round, when their natural flowering time is in October (only!)




                                        Temperature          []

Photosynthesis is a series of chemical reactions and so is dependent on temperature. As always,
more kinetic energy means more collisions and so more enzyme-substrate complexes are formed
                                                       o
and the action goes faster – up to the point (35 – 40 C) when the enzymes are denatured. BUT
– and it is a big but – that assumes that all other factors are optimal. In the real world, this is not
   the case – CO2 is always limiting in summer and day-length and temperature are limiting in
                                                                                o
   winter. So how warm should you keep your greenhouse? Well, 25-30 C is optimum (for C3
                               o
 plants – for C4 plants it is 10 C higher), assuming you can raise the CO2 level, otherwise growth
                     o             o
  is no faster at 35+ C than at 20 C. In summer, the main problem is keeping the temperature
down. This is where shading is supposed to help (less radiation = less heat absorbed); a better
 solution is more ventilation (use a fan) and pouring water on the floor, when its evaporation will
                                        cool the atmosphere.


        Remember:        Growth (or net photosynthesis) = gross photosynthesis – respiration


But if the temperature rises, so does respiration, and if photosynthesis cannot go any faster (due
to lack of CO2) then net photosynthesis (i.e. growth) will be lower at higher temperatures.

                                                                            o
Since there is no photosynthesis at night, the ideal is warm days (25+ C) and cool nights (10 -
  o
15 C), together with long days (18 hours). This is exactly what the northern parts of the UK and
Canada experience each summer and explains why the highest yield of wheat and barley ever
recorded was in these areas.
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Fertilisers            []


Since the rate of plant growth in usually limited by the availability of mineral ions in the soil, then
adding more of these ions as fertiliser is a simple way to improve yields, and this is a keystone of
intensive farming.




Plants need mineral nutrients as well as carbon dioxide and water for photosynthesis. In addition
to the products of photosynthesis plants also need:


    •   nitrogen (NO3) to make proteins and nucleic acids
    •   phosphate (PO4) to make membranes, DNA and ASTP
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    •
                       ++
         calcium (Ca ) to make vitamins and the middle lamella
    •    In addition, they need other soluble ions from the soil too.


Macronutrients are used in relatively large quantities i.e. Nitrate, Phosphate & Potassium (NPK)


Micronutrients are needed in very small amounts, e.g. iron, magnesium, sulphur. If plants lack
these nutrients they show specific deficiency symptoms.


                                 Macronutrient deficiency symptoms:


Element                                 Function in plant               Deficiency symptoms
                                        growth, proteins & nucleic
Nitrogen                                acids                           stunted growth, yellow leaves

                                        nucleic acids, ATP,             poor root growth, blue-green
Phosphorus
                                        membranes                       colour to leaves

                                                                        Poor flowering; susceptible to
Potassium                               enzyme activator                disease;
                                                                        brown edges to leaves
Iron                                    manufacture of chlorophyll      white veins in young leaves
                                                                        yellowing with green veins on
Magnesium                               contained in chlorophyll
                                                                        old leaves
                                        amino acids (and flavours in    yellowing and stunting of plant
Sulphur
                                        onions, garlic etc)             in spring


When plants are harvested the nutrients are removed with them. In a natural ecosystem the
plants would eventually die and decay, with the nutrients being returned to the soil. Farmers need
to use fertilisers containing these nutrients to maintain productivity. Farmers can use organic
fertilisers or inorganic fertilisers.


The most commonly used fertilisers are the soluble inorganic fertilisers containing nitrate,
phosphate and potassium ions (NPK). Inorganic fertilisers are very effective but also have
undesirable effects on the environment. Since nitrate and ammonium ions are very soluble, they
do not remain in the soil for long and are quickly leached out, ending up in local rivers and lakes
and causing eutrophication. They are also expensive.


An alternative solution, which does less harm to the environment, is the use of organic
fertilisers, such as animal manure (farmyard manure or FYM), composted vegetable matter, crop
residues, and sewage sludge. These contain the main elements found in inorganic fertilisers
(NPK), but in organic compounds such as urea, cellulose, lipids and organic acids. Of course
plants cannot make use of these organic materials in the soil: their roots can only take up
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inorganic mineral ions such as nitrate, phosphate and potassium. But the organic compounds can
be digested by soil organisms such as animals, fungi and bacteria, who then release inorganic
ions that the plants can use (refer to the nitrogen cycle). Some advantages of organic fertilisers
are:


       •   Since the compounds in organic fertilisers are less soluble than those in inorganic
           fertilisers, the inorganic minerals are released more slowly as they are decomposed. This
           prevents leaching and means they last longer.
       •   The organic wastes need to be disposed of anyway, so they are cheap. Furthermore,
           spreading on to fields means they will not be dumped in landfill sites, where they may
           have caused uncontrolled leaching.
       •   The organic material improves soil structure by binding soil particles together and
           provides food for soil organisms such as earthworms. This improves drainage and
           aeration.


Some disadvantages are that they are bulky and less concentrated in minerals than inorganic
fertilisers, so more needs to be spread on a field to have a similar effect. They may contain
unwanted substances such as weed seeds, fungal spores, heavy metals. They are also very
smelly!


Increasing the amount of fertiliser increases yield, but only up to a point:




                                                                  This graph shows the results of
                                                                  field trials using wheat – clearly,
                                                                  little is gained after in initial use
                                                                  and heavier seed density is
                                                                  actually counter-productive:
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                            Problems with fertilisers

                                             Leaching     []




                                                                If the nutrients in fertilisers are not taken up
                                                               by plants their is a danger that they will be
                                                               washed out of the soil by rain water and that
                                                               the run off will enter stream and rivers. This
                                                               process is could leaching. The problem with
                                                               this is that it may cause eutrophication.




              Eutrophication            []



This is the process that takes place when freshwater
  is 'enriched' by nutrients, especially nitrates and
                     phosphates.


  The aquatic ecosystems naturally progress from
being oligotrophic (clean water with few nutrients and
algae) to eutrophic (murky water with many nutrients
and plants) and sometimes to hypertrophic (a swamp
 with a mass of plants and detritus). This is in fact a
common example of succession. In the context of pollution “eutrophication” has come to mean a
 sudden and dramatic increase in nutrients due to human activity, which disturbs and eventually
     destroys the food chain. The main causes are fertilisers leaching off farm fields into the
  surrounding water course, and sewage (liquid waste from houses and factories). These both
      contain dissolved minerals, such as nitrates and phosphates, which enrich the water.


Subsequently, this may lead to excessive plant growth (‘algal blooms’) and this, in turn, can lead
to the deoxygenation of the water and the death of much of the animal life. In cold Arctic waters
this does not happen, though the water may still have high mineral content and so be eutrophic.
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 The main source of nitrogen is farming – either nitrates (from fertiliser over-use) or urea (from
 over-use of slurry/manure). Correct timing of fertiliser application can reduce these problems.
Cold soils prevent crops absorbing nutrients, so there is no point applying them in winter or cold
 spring weather. Heavy rain leads to the nutrients being washing into groundwater before the
     plants have had a chance to absorb them; so do not apply if heavy rainfall is forecast.


  The main source of phosphates is sewage. Phosphates are added to detergents to improve
   washing performance – particularly in hard water areas. Sewage is also warm too, so the
combined effect on plant growth downstream is quite marked. Aeration of the sewage outfall (by
 weirs or spraying) will increase oxygen levels and improve water quality for animal life, but do
                         nothing to the mineral load added to the water.
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                        Chemical Control of Pests                              []



To farmers, a pest is any organism (animal, plant or microbe) that damages their crops. Some
form of pest control has always been needed, whether it is chemical (e.g. pesticides), biological
(e.g. predators) or cultural (e.g. weeding or a scarecrow). Chemicals pesticide include:


    •      herbicides        anti-plant chemicals
    •      insecticides      anti-insect chemicals
    •      fungicides        anti-fungal chemicals
    •      bactericides      anti-bacterial chemicals


Pesticides have to be effective against the pest, but have no effect on the crop. They may kill the
  pests, or just reduce their population by slowing growth or preventing reproduction. Intensive
farming depends completely on the use of pesticides, and some wheat crops are treated with 18
  different chemicals to combat a variety of weeds, fungi and insects. In addition, by controlling
  pests that carry human disease, they have saved millions of human lives. However, with their
    widespread use and success there are problems, the mains ones being persistence and
                                          bioaccumulation.




                   Competition between crop plants and pests

           Weed species compete with the crops for light, space, water and nutrients.


Flax is grown for its oil – linseed – as well as for its fibre, used to make linen. It is not common in
  the UK, but has pale blue flowers, so is readily identified in early summer. A common arable
weed is known as Wild Oat, which competes with it. Wild Oat seeds ripen earlier than flax and so
        carry forward to the next crop. The effect of Wild Oat on flax yield is shown below:


                          Fertilised plots, yield     Unfertilised plots,
  Wild Oat density                                                             Average reduction in
                                  of flax               yield of flax
      (no./m2)                                                                      yield (%)
                               (tonnes/ha)               (tonnes/ha)
           0                       19.5                       17.9                        -
          10                       13.4                       14.3                       26
          40                        6.7                        8.0                       60
          70                        4.3                        6.3                       72
         100                        3.5                        4.2                       80
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            130                       3.4                        3.4                       82
            160                       2.9                        2.3                       86

        Like all crops these days, farmers normally use ‘direct drilling’, spraying the land with
  glyphosate (= ‘Roundup’), a weed-killer that kills all germinated weeds and allows the crop to
 germinate in weed-free conditions. ‘Tram-lines’ are used to confine wheel-tracks to the smallest
                   possible area. In addition, ploughing is no longer used, since it:


    •     exposes new weed seeds to the air, encouraging them to germinate (WWI poppies?)
    •     allows air into the soil, increasing oxidation of desirable organic matter, thus reducing it
    •     disrupts worm channels, reducing water penetration and drainage


‘GM’ crops have been modified to allow this weed-killer to be used on the growing crop too (it has
no effect on seeds), thus further improving crop yield and Monsanto’s profits!


   Animal pests (mainly insects) damage crops by feeding on them. They lower crop yield by:


              1. Eating their leaves and so reducing photosynthetic area (e.g. caterpillars)
    2. Sucking their sap (e.g. aphids or greenfly). This removes the products of photosynthesis;
              however, the main problem with these insects is that they act as vectors for virus
                                                    diseases.


Use                    of                   chemical                    pesticides                       []

Insecticides act either by contact or are distributed inside the plant (systemic) and are then
eaten by the insect. The latter are a more recent development and have the advantages that only
harmful insects are killed and rain does not wash the chemical away, so less spraying is needed
and young plant growth is continually protected.


There are four main modes of action of insecticides:


    •     Contact Insecticides-These chemicals require the insect to touch the insecticide (e.g.
          DDT). The insect is either sprayed directly or walks through deposited spray.
    •     Systemic insecticides - Sap feeding insects are particularly vulnerable to these
          chemicals. Sprayed material is absorbed by the plant, entering the phloem. When the
          aphid feeds on the sap, it withdraws poisoned fluid.
    •     Stomach ingestion insecticides - These compounds are sprayed over the crop so that
          those pests with biting mouth parts, like flea beetles, caterpillars and weevils, eat a
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        poisoned meal. These pests are usually too big to be much affected by systemic
        insecticides.
    •   Fumigant insecticides - Vapour given off by the insecticide is inhaled by the insect.
        Fumigants are mainly used for soil treatment or in grain stores, where the chemical
        vapour can give optimum penetration e.g. Vapona


Problems                          with                         insecticides                      []

Some pesticides are persistent so do not break down in the environment or within the tissues of
living organisms. This gives rise to two potential problems:


    a) Bioaccumulation - the accumulation of a substance in living tissue. Organisms at any
    trophic level may be capable of bioaccumulation.


    b) Biomagnification is the increasing concentration of a substance up a food chain - i.e.
    from one trophic level to the next . Animals at the higher trophic levels will be most affected
    (e.g. us!).


Example
Some of the earliest insecticides were organochlorines, such as DDT, which kills by both contact
and stomach ingestion. DDT was the first known contact insecticide, synthesized in 1874. It has
saved more human lives than any other chemical; it widespread use against mosquitoes briefly
eradicated malaria in many parts of the world (resistance rapidly built up, however, and Malaria (=
‘bad air’) is once again on the increase).         Unfortunately, DDT persists in fatty tissues
(bioaccumulation). Larger, long lived predators at the end of a food chain may accumulate a
lethal quantity of DDT as a result of eating large numbers of smaller species (biomagnification).


These dangerous effects coupled with the appearance of resistant insects soon led to the
banning of DDT and related compounds (e.g. Lindane) except in specialised environments, such
as wood-worm treatment of loft timbers. Even that use has now ceased in the EU.


                        Water   zooplankton    small fish large fish  birds
                                 0.04ppm    0.5ppm 2ppm         25ppm
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     Sparrow hawks are long lived and eat large numbers of smaller species in their lifetime.



                                        Fungicides      []



Fungi are the main plant pathogens. Able to grow in a wide variety of conditions and able to
produce cellulose to digest cell walls they cost farmers and growers millions each year.


                                                                         In 1848, potato blight
                                                                      (Phytophthora        infestans)
                                                                      came to Ireland, where the
                                                                      staple crop of the local
                                                                      people was potatoes. Over
                                               the next few (wet) summers, it spread and
                                               destroyed the crop, for which there was then no
                                               cure. The Irish resented growing corn, which was
                                               unaffected and shipped abroad, when they were
                                               starving and riots followed. In 5 years, half the
  people preparing to leave Ireland forever    population of Ireland starved or emigrated (mainly
          during the potato famine             to the USA – as the Kennedy and Clinton families
did). Much of the bitter hatred of the English that still exists today in ten conflicts of Northern
Ireland have their origins in the effects of a fungus! By 1852, the use of Bordeaux Mixture had
transformed the outlook and potatoes could again be grown with success.


Blight turns the leaves of the plant brown and the plant diverts all its sucrose energy to the
infected leaves, rather than to the growing tubers. After a few weeks, the whole of the plant is
infected and it dies.
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           The tubers become infected too and turn to a brown, stinking, inedible mess.

Traditional fungicides – e.g. Bordeaux Mixture. These are based on heavy metals (e.g. copper,
manganese), together with a ‘sticking’ agent so that it stays on the surface of the leaves. These
chemicals are   purely preventative, and must be sprayed on the crop before
infection. They have the advantage that resistance cannot build up, but the
drawback that they are washed off when it rains and that new growth is
unprotected.       They can, of course, be easily washed off the food before
consumption.       Some other old-fashioned remedies were based on tar-oil compounds (e.g.
creosote) and their impact on the environment can only be guessed at – certainly, some of their
ingredients are carcinogenic!


Systemic insecticides – e.g. Benlate. These are systemic in their action i.e. they are absorbed
and carried around the plant. In addition to being preventative, they are also curative (at least in
part) and so preventative spraying can be reduced. They also protect new growth as it appears
and so, again, less spray is needed. The old idea of ‘preventative spraying’ has long been
abandoned – it merely leads to the build-up of resistance. They have the drawback that all the
washing in the world cannot remove them from our food. In the past few years, nearly all the
fungicides available to the gardener have been withdrawn from the market.


Biological control of pests                          []




1. What are the characteristics of pest species?

They cause economic damage (e.g. eating growing crops, stored crops or damaging buildings) or
  have health implications (e.g. vectors of disease). Many are capable of very rapid population
 growth. Often the normal factors which would regulate their numbers (i.e. natural enemies) are
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 not present. This may be because they have been imported into a part of the world where their
  natural enemies do not exist, or because these enemies have been suppressed in some way
                                       (e.g. use of insecticides).


                     2. What is the aim of biological control?

       This is not always easy to determine. For example if you live in a house infested with
  cockroaches your pest control aim might be to eliminate all of them. However, in general it is
 accepted that the aim is to depress the pest population below the Economic Injury Level (EIL):-
   That is where the costs of the control measures start to exceed those of the extra revenue.




                                                              The graph shows an idealised situation. The
                                                              introduction of a control measure depresses the pest
                                                              population size below the EIL, but does not eliminate
                                                              the pest completely.




                3. What is a suitable biological control agent?

                          Introduced control organisms should preferably:


                                            •     reproduce rapidly
                                        •       be specific to the pest
                                   •    have good searching capacity


                                  so that they keep the pest at low numbers


If the pest species is alien (i.e. not indigenous) then the following programme is usually followed:


  i.    Search in the pest species native country for a suitable organism
 ii.    Identify if the geographic, climatic and political conditions are right for release
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 iii.   Quarantine the organism, rear the control organisms for release to ensure any unwanted
        parasites are not introduced with it.
 iv.    Select a suitable release area.
  v.    Extensively trial the proposed control agent, within a contained environment
 vi.    Seek EU Commission approval (UK government not allowed to)
vii.    Convince Greenpeace etc that what you are proposing is safe
viii.   Market the proposed product




4. What groups of organisms can be used for biological control?

        1. Insect parasites These have the advantage that they are generally specific in the host
        on which they lay their eggs. The larvae eat the host from the inside once the eggs
        hatch.


        2. Predators. These are carnivorous and so may, in turn become a pest if, having
        reduced the original pest to a low level, then attack other species e.g. Cane Toad in
        Queensland, Aus.


        3. Pathogens The best known example is the bacterium Bacillus thuringiensis, the toxin
        from which kills a wide range of caterpillars. If a virus is used, they are generally specific
        in action.




5. What examples of biological control are there?

        1.       The control of White Fly by the parasitic wasp Encarsia formosa in greenhouses.
                 This is now widely used as an alternative to pesticide control.


        2.       The control of water hyacinth, which was introduced to the USA from S. America,
                 by a weevil.


        3.       The use of grass carp in the UK to control weed growth in ponds and waterways.
                 It is claimed that water temperatures in the UK are too cold for them to breed.
                 However, the use of an exotic species such as this is still extremely controversial.
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Can things go wrong? Yes, especially if a non-native species is used. On Granada mongoose
were released to control the rat population, however, they have had a devastating effect on native
species- especially some ground-nesting birds.




Advantages and disadvantages of biological control.

Advantages                                       Disadvantages
1. It should not intensify or create new pest
                                                 1. Control is slower.
problems – the organisms used are selective.

2. No manufacturing of new chemicals: the
organisms are already available and so           2. It will not exterminate the pest.
‘organic’.
3. Control organisms will increase in number
and spread.                                      3. It is often unpredictable.

4. The pest is usually unable to develop
                                                 4. It may well require training in its use.
resistance.

                                                 5. It is difficult and expensive to develop and
5. Control is largely self-perpetuating          supply – and where’s the developers’ profit?
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Integrated Pest Management                                  []



It is now recognised that the most cost-effective form of pest control is achieved by using several
different approaches together: a combination of the following is known as Integrated Pest
Management.




                                         Crop rotation

Fields of most crops attract pests in large numbers. If the same crop is grown on the same piece
of ground year after year pests persist in crop debris, in the soil or in hedgerows from one year to
  the next, with the result that infestations increase in severity with yields and quality suffering.
   Crop rotation has been widely practised from the earliest times. Today, one common 4-crop
rotation is: cereal A; legumes, cereal B; oilseed rape or other ‘break’ crop, following one another
 in a regular sequence. This limits the population density of pests and diseases, especially those
           with annual life cycles associated with specific crops – such as potato blight.




                                 Pest-resistant varieties

  Some varieties of crop plants are naturally resistant to certain pests and diseases. For many
 years selective breeding has been used to transfer the genes that confer this natural protection
  into high-yielding strains. More recently, genetic engineering (using recombinant DNA) has
served a similar purpose. The transfer of a gene across the species barrier from the bacterium B.
thuringienis into cotton is an interesting example of conferred insect resistance – and most cotton
  is now ‘GM’. In the future, more pest-resistant varieties should result from the applications of
                                           biotechnology.




                              Biological Control – see notes



                                  Pesticide application
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    The primary aim of integrated pest management is to hold pests and diseases below the
Economic Injury Threshold. Providing this can be achieved by other means, pesticide application
can be held in reserve. However, if a farmer receives advance warning of an impending increase
in a particular pest or disease, effective control can usually be achieved by applying a pesticide at
the optimum time. The old-fashioned practice of ‘preventative spraying’ was both costly and an
                             invitation to the pest to become resistant.




                                                  []



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