Nitrogen & Phosphate

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Nitrogen is a component of amino acids , the building blocks of proteins. The N content of most proteins varies between 14 and 18%. Plants need nitrogen primarily for leaf development. The amount of N taken up by crops directly affects the size of the green canopy (total LEI) through which plants absorb sunlight and CO2 for photosynthesis. Cereals use around 30kg of N to form 1 ha of green canopy, this is nearer to 40 kg for sugar beet and potatoes. The Earths atmosphere is approximately 78% nitrogen existing as an inert gas which the majority of living organisms cannot exploit. Nitrogen, therefore has to be fixed before use. Farmers use both organic manure and mineral nitrogen on their crops. The main problem with nitrogen is not so much the cost of the input but the ease in which it can be lost from the system. Nitrogen is the only crop nutrient that can be lost in significant amounts to the atmosphere in gaseous form . It is also very easily lost in its soluble form as nitrate ions via leaching. 

The Nitrogen Cycle

A convenient way of visualizing the movement of nitrogen is to identify the internal pathways (in the farming system) and the many sources of loss an addition of nitrogen to the system. Many different graphical representations of the  N cycle have been published, the one below is a fairly simple representation, covering all the key points.

 

Adapted from Solving the Nitrate Problem. MAFF (1996)

Nitrogen in the soil exists in organic compounds bound within the soil complex. These compounds are almost completely unavailable to plants. In the inorganic form they are available as ammonium (NH4+) and nitrate (NO3-) ions. The positively charged ammonium ion is relatively stationary and is absorbed by organic matter or clay particles. The nitrate ion is negatively charged and is therefore not held to the other negatively charged soil collides and is therefore mobile in the soil solution.  Soil bacteria, fungi and actinomyctes are responsible for the conversion of N from one form to another. The activities of these micro-organisms, and therefore the form of nitrogen available to crops is profoundly effected by cultivations, organic additions (manure), crop rotation and root exudates. One of the main features of the N cycle is the turnover of soil N by:

bullet Mineralisation - The conversion of bound organic nitrogen into the mineral (ionic or inorganic) form required for plant uptake. The inorganic nitrogen is then converted in a process known as nitrification  from ammonium salts to nitrites, and from nitrites to nitrates by nitrifying bacteria which obtain their energy from this oxidation process. Once free nitrate is formed, the rapid recycling process offers many options, the nitrate may be immobilized, assimilated by plants, denitrified or leached. 
bulletImmobilization - This occurs when inorganic ions are assimilated by soil organisms and are bound organically again. 

These processes occur simultaneously in a continuous cycle. It is the net movement in one direction that determines if more or less inorganic nitrogen results and this is highly dependant on the soil organic matter content.

Pause for thought........List 4 reasons why it is important for arable farmers and managers to have at least a basic understanding of chemistry and biology. 

Nitrate Leaching

Forage maize crop at field capacity and subsequent run off into water courses, but what is the nitrate level of the water.

The movement of water, and hence nitrate, from the soil to groundwater is influenced by:

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The depth of the aquifer and the characteristics of the rocks forming it. 

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Water will pass very quickly through fissured rocks such as limestone, and less quickly through a fine matrix such as sandstone or chalk. 

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Dilution (drainage volume - see chart below) can have an effect on the amount of nitrate present in groundwater; higher rainfall areas tend to have lower nitrate concentrations in groundwater hence:

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Some areas of country at higher risk than others 

 

 

Source MAFF (1991)                  

Losses of nitrate are dependent on:

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Type of farming system in operation, in turn dependent on:

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Nitrogen inputs from fertilizer, manures and feedstuff

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Nitrogen outputs, harvested crops and animal products

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Soil type:

 

Source: MAFF (1996)                  

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Crop type:

 

Source: MAFF (1996)                  

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Crop cover:

 

Source: MAFF (1996)

             
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Crop rotation: From the chart below it can be observed that nitrate losses are far greater in the arable years than the grass/clover years

Source: Davies & Barraclough (1988)           
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Time of year

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Weather patterns

Pause for thought...... Use the above information to suggest a 5 year rotation that could be used to minimise nitrogen leaching. 

Therefore:

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Nitrate concentration at point of extraction depends on overall balance of agriculture in catchments area and:

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Maximum allowable nitrate concentration in drinking water is 50mg/l (EC Nitrate Directive 1991).

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Nitrates in drinking water have often been blamed for Blue Baby Syndrome

Nitrate Levels in River water

River pollution depends on geographic location and flow rates.

Very few surface abstractions from rivers exceed the 50 mg/l EC limit but this depends on: 

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Time of the year:

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Peak concentrations usually occur in late autumn when there are larger areas of fallow ground and run-off is at its peak

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If groundwater contribution to rivers is low then the autumn peak is less obvious

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On predominantly clay soils, especially where there is a high concentration of land drains, rivers are likely to show a definite autumn peak after the first substantial rains

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Nitrate concentration in rivers also depends on the characteristics of the catchment area, i.e. topography, slope etc.

Other factors include:

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Land drainage 

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Cultivation and reseeding

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Grazing and machinery operations 

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Fertilizer and manure applications

 

Pause for thought........ Is it sensible for farmers to change their farming systems in response to a single issue like nitrate concentration?   Nitrate could be extracted downstream or the concentration lowered by adding "clean" water.    What are the advantages and disadvantages of the latter approach?       

Summary - Nitrates

No agricultural system can be 100% efficient in its use of nitrogen, nitrate leaching is a natural process and some loss is inevitable. Most systems can be improved resulting in a reduction in the quantity of nitrate lost each winter.  Two particular practices can result in excessively high leaching from any farming system:

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Nitrogen fertilizer used in excess of crop requirement and potential crop uptake

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Research has shown that increase in applications in autumn when crop uptake is low can lead to increased levels of leaching (chart below).

 

Source: MAFF (1991)                  

The trend is, however, promising:

Source: MAFF (1991)            

              

bulletApplication of  animal manures, sewage sludge's and other organic wastes in excessive amounts and /or inappropriate times
bulletIn the last 25 years there has been an overall fall in Nitrogen application rates in Great Britain.  Annual rates have fallen from over 140kg/ha in the early 1980s to 105kg/ha in 2007 (British Survey of Fertiliser Practice, 2007).  The amount added to tillage over this period has however remain constant at around 150kg/ha, while the amount added to grass has declined from around 130kg/ha to 65kg/ha in 2007. For more details CLICK HERE for the BSFP report.

Pause for thought.......List 4 methods of minimizing soil nitrate losses

CLICK HERE For a nitrogen response crop growth model. Select an area of world from the table, vary the inputs and run the model, look at tabular and graphical responses. Try it for different parts of the world, and vary the data inputs before running model. Try it also for other nutrients, i.e. potassium

Changes in Nitrate Vulnerable Zones

In October 2002, 47% of England was designated as a Nitrate Vulnerable Zone (NVZ), this was in addition to the 8% originally designated in 1996.  This was due to the a rethink on the Nitrate pollution policy by the UK Government following a ruling by the European Court that insufficient areas had been designated.  Maps and information on nitrate pollution and the designated areas can be found on the special DEFRA website which also allows identification NVZ's by postcode.  Extending NVZ's to cover 70% of England is currently under review, with designation occurring sometime in 2008.

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Phosphorous

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Essential plant nutrient for enzymetic reactions that depend on phosphorylation to make energy available for biochemical reactions

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Deficiency in plants cannot be compensated for at a later growth stage

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Since 1945 soil P levels have built up in many areas as farmers have endeavoured to improve soil fertility

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Phosphate is relatively insoluble (unlike nitrates) therefore losses tend to be associated with soil erosion in particle form

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In soils which have high phosphate levels there can be a loss of soluble phosphates in drainage water

 

Source: Soil and Land Research Centre/National Soils Inventory (1998) 

From the chart above:

bulletPhosphorus levels in topsoil increased between 1979 and 1985
bulletPhosphorus available to plants decreased owing to:
bulletIncreased use of organic and inorganic fertilizers and increased livestock stocking rates
bulletAvailable phosphorus replenished from soil reserves (unavailable) at a very slow rate
bulletUnavailable phosphorus is prone to losses

Therefore:

bulletMatch P applications to crop needs
bulletSoil analysis

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