Integrated Crop Management (CPA Book)

Soil Management and Crop Nutrition

Why does the soil need to be managed?

The soil is a living medium and a fundamental natural resource. Careful management is paramount, because without it many of the other inputs to an ICM plan will be negated.

Soil is a dynamic biological system that provides plants with mechanical support, space for root growth and development, oxygen for respiration, water and nutrients for growth and a medium for interaction with other organisms. Soils vary enormously in their natural fertility but in a natural, unmanaged state all soils gradually become more and more acid and the nutrients are depleted.

...replenishing nutrient losses

Whenever any crop is harvested or grazed some of the nutrients used to grow it are taken too. This, of course, includes animal products as well as crops. Additionally, some nutrients, especially nitrogen, are lost to the air or through the soil to groundwater by leaching. Even where a large proportion of the nutrients are recycled in the form of farmyard manure or slurry, there is still a net loss of fertility. Nutrient management thus forms an important part of the overall soil management plan.

...optimising the use of available resources

In farming terms, long term experiments have shown that yields obtained without any artificial replenishment of fertility are well under half the optimum yield from a properly managed system. It obviously makes sense, economically and environmentally, to make efficient use of crop and animal wastes, thereby reducing the cost of, and dependence on, manufactured fertilisers. But it is also necessary to adopt farming practices that minimise nutrient losses in the first place, especially where the loss then becomes a pollutant.

...protecting water

Manure or fertiliser in water causes a rapid increase in the population of algae and other higher plants which take advantage of this food supply. In doing so they use oxygen dissolved in the surface water and, in extreme cases, all the available oxygen can be removed, This enrichment of water by nitrogen or phosphorus is called eutrophication and it has a serious effect on water quality.

Other dangers to aquatic life include contaminants such as pesticides or heavy metals which may be present in organic manures, leakage from slurry lagoons and effluent from silage clamps. Storage, maintenance and siting of all of these potential sources of pollutants needs careful management. The impact clearly depends on the extent of the contamination and the dilution in the water, but sudden inputs can kill large numbers of fish.

Soil management includes calculations of the risk of run-off from the land into watercourses. The speed at which soils soak up liquid is important in working out this risk. water ponding on the soil is a clear indication that it is being applied (where as rainfall or by the grower) faster than it can be absorbed, and the risk of run-off is clearly heightened, especially on sloping ground. Paradoxically, fields with effective land drainage systems create a particular risk because of the higher speed at which liquids can reach watercourses.

The potential for damage, in a badly managed system, to watercourses and the life they support is therefore considerable.

Planning for better soil management

Planning soil management starts with a detailed knowledge of the soil on the farm which should be assembled by creating a soil map. This is information that is specific to the site and it is one of the fundamental reasons why ICM can never by a prescriptive set of rules.

The DEFRA Code of Good Agricultural Practice for the Protection of Soil is a practical guide to help farmers and growers avoid causing long-term damage to their soils. It also provides general guidance on practices which will maintain the ability of the soil to support plant growth.

...finding out about the soil on the farm

Ideally farmers need to be able to identify their main soil series because these are groupings of soils that respond to management in a consistent and predictable way. Each soil series is given a name, normally that of a place near where it is commonly found and usually where it was first recognised and described. For example, the Evesham series are lime-rich soils formed from clays and found in Warwickshire, Gloucestershire, Somerset and the East and West Midlands. They are normally very heavy soils best suited to grass and cereals.

Professional advice is available, but there are also soil maps available from which farms can identify the soil series likely to be present on their land. There are about six most likely to be present, and the top 38 series cover 80% of the farmland in England and Wales.

...making a farm soil map

An accurate map provides site-specific information about the soil's water holding capacity, its workability, drainage and waterlogging, acidity and natural fertility. More refined mapping techniques can provide information to show how quickly nutrients and chemicals can leach through the soil. The map forms the basis of an action plan.

Soils vary from field to field, and often within fields. The scale of the map may be governed by the size of the farm but a sampling frequency of one point for every 2 ha of land is a workable guideline. From what has been said it is clear that the map should contain qualitative and quantitative information covering things such as texture and pH, structure, organic matter content, top soil depth and areas which will require special treatment, perhaps because of the presence of pans or impermeable layers.

...soil fauna and flora

Soil life is abundant and diverse. It contains not only visible organisms such as earthworms and a variety of living insects, but also literally millions of micro-organisms. These include nematodes, algae, fungi, protozoa and bacteria. Estimates of the mass of microbes in soil range from 800 to 3000 kg dry weight per hectare in the top 20cm of soil. Not all soil organisms are beneficial and the abundance of those that are pests, such as nematodes, is heavily influenced by cropping rotation.

Other factors that can have a bearing on soil populations are soil type, temperature, water supply, the concentration of some heavy metals and the use of pesticides.

Soil fauna and flora

"It has been estimated that there are one billion bacteria, one thousand metres of fungi and thousands of tiny animals and algae in a thimbleful of fertile soil. This is equivalent to one tonne of microbes per hectare of soil."

           Bill Butterworth


Micro-organisms play a vital role in maintaining soil health by processing (mineralising) organic matter to liberate nutrients for use by growing plants. The process produces humus which contributes to creating the soil structure.

Clearly a soil management plan must aim to integrate maintenance of the physical structure with protection of the fauna and flora. A single act of cultivation may be damaging to the latter as it beneficial to the former.

...defining texture

In addition to the fertility status of soil, its physical texture is also vitally important. All soils consist of a mixture of different sized particles which are classed as sand, silt or clay, according to their size, together with organic matter and a variety of soil-living creatures. The amount of organic matter in mineral soils is typically 0.5-2.5%, but on fen or peat soils the proportion can be as high as 40-50%. At these levels the behaviour of the soil, and in particular the capacity to adsorb chemicals, is significantly changed. Also although rich in nitrogen, black fen soils are generally deficient in other essential crop nutrients.

Field determination of soil texture

By rubbing a moist sample of soil between thumb and fingers it is possible to classify the texture of the soil according to the 'feel' of the dominant particles.

  • Clay is sticky, will take a polish and forms a ball that can be moulded with difficulty;

  • Silt feels silky and smooth, will take a slight polish and forms a cohesive ball that can easily be moulded;
  • Sand feels gritty and can be heard if rubbed close to the ear, it forms a less cohesive ball that is easily deformed.

Soil particles are classified into categories according to their size:

Clay   < 0.002 mm

Silt     0.02 - 0.002 mm

Sand  2.0 - 0.02 mm


The texture of a soil depends on the relative proportions of the different particles and organic matter. There is a relatively simple method by which a competent advisor can determine soil texture using the DEFRA Soil Texture (85) System. This can influence decisions about pesticide choice and dose. texture is also a guide to the ease of cultivation of a soil.

...the importance of soil structure

Soil structure is the result of the way in which soil particles aggregate together. It depends on all the properties (mentioned above) that determine texture, together with chemical factors and the activity of organisms such as earthworms. The structure of the soil has a major influence on the ease with which water and air move about in it, and on moisture retention. These factors affect crop growth and the availability of nutrients but they also influence what happens in periods of very dry or very wet weather. A normal arable soil is regularly cultivated to a depth of 20-30 cm which is the main area for crop root development, although most crops can develop roots well below this level for the uptake of water and nutrients. Problems occur on some soils if a crop is established at a time of plentiful supply of water and nutrients in the surface layers. Root growth develops only in these layers and may not be sufficiently deep to cope with later drought conditions.

Improving the physical condition of the soil

Maintenance or improvement of the soil structure is an important part of any farming system. The physical condition of the soil is most affected by cultivation techniques, machines and soil water management. The DEFRA Codes of Good Agricultural Practice for soil, water and air, together with the SEERAD Code of Good Agricultural Practice for the Prevention of Environmental Pollution from Agricultural Activity all give valuable guidance.

...matching resources to needs

It is self-evident that the amount of work that can be done is limited by the number of available work days and the available manpower. However, it is much better to estimate these in advance and plan ahead, rather than discovering where the bottle necks occur mid-season. The plan should not only schedule the work to be done, but it should also be a strategy for minimisation of damage to the soil structure.There are various measures that can be taken to achieve this including matching tractor power to the job and using special equipment such as dual wheels, flotation tyres or tracks. Regular maintenance of all vehicles, with particular attention to tyre pressures is an integral part of the overall plan.

Field operations must be matched to the crop and the prevailing soil conditions. This involves actually measuring soil moisture and compaction by digging a hole with a spade and examining the exposed profile for evidence of compaction or a plough pan. From these measurements and the weather forecast, decisions can be made about the best time and method for cultivation. Here, as with everything in ICM, there is no hard and fast rule to follow, and sometimes different aims will make conflicting demands. For example, non-inversion cultivations require less energy than ploughing and do less damage to the soil fauna. On the other hand, this will encourage annual grass weeds and it is less beneficial than ploughing if the structure needs to be improved. An Integrated Crop Management system means exactly that: all needs must be considered, and the eventual action should be the result of a balanced assessment. In this case it is likely that the best course will be alternate ploughing and non-inversion techniques in the rotation.

...soil water management

Soil water management is concerned with the conservation of moisture at certain times, its removal at others and the prevention of soil erosion. The two main factors involved in erosion are wind and water, and the extent of the problem varies according to soil type, location, slope, field size and crop husbandry. In severe cases erosion can lead to substantial crop losses and water pollution. The measures necessary to prevent this on any farm will vary, but the overriding principle should be to avoid leaving the soil bare. in some countries there is a legal requirement to have green cover of some sort during the autumn months. This is not the case in the UK, but nevertheless it is a sound principle. The timely establishment of autumn crops not only stabilises the soil during the wet winter months, but it also plays an important part in preventing nitrogen leaching. Leaving an uncultivated grassy buffer strip alongside watercourses assists in avoiding direct pollution by fertiliser or pesticides, but it also protects the water from direct run-off or wash-down of soil.


Heavy metals in soil

Maximum permitted concentrations (mg/kg dry matter) of metals in soil after application of sewage sludge.

 Soil pH

Metal 5.0-5.5 5.5-6.0 6.0-7.0 >7.0
Zinc 200 250 300 450
Copper 80 100 135 200
Nickel 50 60 75 110

Source: MAFF Code of Good Agricultural Practice for the Protection of Soil


Organic manures are a valuable means of building up soil organic matter which assists water retention and protects against erosion. However, liquid organic wastes require special care in order to avoid soil pollution from the possible presence of heavy metals, and by nitrate leaching. The risk is higher from animal manures and organic wastes than it is from inorganic fertilisers. Nitrogen application should never exceed the crop requirements and the maximum amount of nitrogen from organic manure should be 250 kg per hectare in any year. On sloping land or near domestic properties where there is a risk of water or air pollution, organic manures should be incorporated into the soil as soon as they are spread.

Crop nutrition management

Plants depend on mineral nutrients for their growth and development. there are 13 elements (see table) which are essential for all plant growth and a further four or five which are beneficial for some plants.

The six major nutrients are nitrogen, phosphorus, potassium, sulphur, calcium and magnesium. The first three of these are needed in the largest amounts and are normally referred to colloquially by their chemical symbols N, P and K, although they are always utilised as compounds of these elements. Plants can only take up these nutrients as water soluble compounds and it is often the case that while the topsoil content of nitrogen may be relatively high, only a small proportion is available following release in a complex web of interrelated biological and chemical breakdown. This is known as the nitrogen cycle.

Plant nutrients

Nutrient and symbol

Form in which taken up by plants

Removal by 5t/ha @ 20% m.c. (kg/ha)


Nitrogen (N)

NH 4+,NO3-


Phosphorus (P)



Potassium (K)



Sulphur (S)



Magnesium (Mg)



Calcium (Ca)



Trace elements:

Chlorine (Cl)



Iron (Fe)



Manganese (Mn)



Zinc (Zn)



Copper (Cu)



Boron (B)



Molybdenum (Mo)





Nitrogen is taken up by plants in a number of ways. Soil organic matter and crop residues are broken down by microbial conversion to soluble forms in a process known as mineralisation. This process occurs whenever the soil is not frozen and in the UK is most rapid in autumn when soils are warm and moist. A second source of nitrogen is obtained from the atmosphere by  a bacterial process known as nitrogen fixation in which it is made available as nitrate or ammonia. Thirdly, nitrogen is obtained from soluble organic compounds such as urea and amino acids, whether from animals or applied as fertiliser. Finally, these sources of nitrogen can be supplemented by manufactured nitrogen fertiliser.

The rate at which crops take up nitrogen varies with the crop, its rate of growth and stage of development. Some crops like cereals have a peak demand during early growth while others, for example potatoes, have a more even uptake during the season. However, care is necessary in treating a backward cereal crop. Although the demand may be high, the ability to take up a heavy nitrogen dressing may be limited by a root-system that is not yet well-developed. In such cases split treatments may have to be considered.

Highly permeable underlying rock like sandstone or limestone will allow rapid movement of excess nitrate through to groundwater. This has led to the creation of a voluntary scheme of restriction of nitrogen use in the UK (Nitrate Sensitive Areas) and statutory areas of control (Nitrate Vulnerable Zones) under the EC Nitrate Directive. This Directive requires restrictions in any area with 'polluted' water; which is defined as any surface or groundwater that contains, or could contain, more than 50 mg nitrate per litre if action is not taken.

Nitrate Sensitive Areas (NSAs):

  • Voluntary scheme covering 32 specified areas in England and Wales on sandstone, limestone or chalk where nitrate contamination of drinking water deemed most likely to occur. Farmers join scheme for 5 years and adopt restrictions on nitrogen inputs, crop cover and organic manures;
  • Premium scheme involves higher payments for converting from arable to grass;
  • Administered by MAFF;
  • Scheme covered 19,600 hectares in 1996.

Nitrate Vulnerable Zones (NVZs):

  • Statutory controls designed to achieve much the same result as NSA scheme;
  • Manufactured N banned between 1 September and 1 February unless there is a specific crop need;
  • Organic manure total quantities limited;
  • Field by field records of N usage compulsory;
  • MAFF grant of 25% available for providing facilities to separate clean and dirty water.

The ultimate result of nitrate loss to groundwater is the contamination of drinking water supplies where the EC Drinking Water Directive also imposes a maximum nitrate concentration of 50 mg per litre.

...the right nutrients in the right amounts at the right time

Like everything else in Integrated Crop Management,   crop nutrition requires planning. Crops need an adequate supply of food and it is clear that, in any seasonal sequence of cropping, the natural fertility of the soil must be supplemented to achieve optimum results. Even soils like light sandy soils which have low natural nutrient reserves, can be highly productive when managed properly, In an ICM system the aims should be to make best use of the natural resources and then supplement them by using the right nutrients in the right amounts in the right place. This demands skill and often professional advice.

The starting point in making the best use of resources is to devise a cropping and soil care strategy that minimises unnecessary nutrient losses. This will include maintaining green cover on the land, especially in the autumn months and keeping cultivations to the minimum. After ploughing in high-residue crops, such as grass, it is important to make sure that the subsequent crop is established quickly to utilise the high nutrient release from the decaying grass before it is lost in the lower levels of the soil and ultimately to watercourses.

...nutrient status

The nutrient status of the soil needs to be determined at the outset and then repeated every 3-5 years. This includes a measurement of the acidity (pH) as well as levels of phosphorus, potassium and magnesium.

The nitrogen index of the soil (on a scale of 0-2) is based upon the previous crop grown in the field. The higher indices are achieved after crops that have received large quantities of nitrogen fertiliser, such as permanent pasture and potatoes, or have created large reserves of nitrogen, such as legumes. Crops like cereals, maize and forage crops, that are removed from the field, generally result in a Nitrogen Index of 0. Mineral nitrogen analysis helps in the diagnosis and prevention of nutritional disorders in crops. For arable soils samples should be taken with a screw auger to a depth of about 15cm (6"). At least 25 sub-samples should be taken from each sampling area (about 6 hectares (25 acres)), bulked and sent to a specialist laboratory offering an analytical service.

Nitrogen, phosphorus and potassium are the major elements whose levels would gradually decrease without replenishment and, assuming satisfactory levels to start with, the aim must be to replace that which is removed so that the nutrient status is maintained. and demand

Farm requirements are worked out by calculating a 'balance sheet' of supply and demand. Predicted demand needs to be based on a realistic yield projection, the current state of the crop and its variety, and the seasonal weather. Supply is calculated from information about previous cropping (the Nitrogen Index), the status of other nutrients and the contribution from any organic manuring. Fertiliser needs to make up the balance can then be calculated.

Whilst some of the judgements must be subjective, they should be based on what is already known. This includes information from the farm soil map, past yields and experience, and rainfall data. Clearly, the more complete these records are, the sounder will be the decisions made. Additional help and guidance is available from publications from DEFRA and fertiliser manufacturers. This includes, for example, nitrogen indices and information on the composition and nutrient value of different animal manures and crop wastes. The DEFRA Reference Book 209: Fertiliser Recommendations for Agricultural and Horticultural Crops is an essential companion for the calculation of detailed crop requirements.

Using fertilisers: choice, type and quality

Fertiliser is either natural or manufactured. Choice of which type to use will, to some extent, be governed by the availability of organic manures from the farm enterprise. These may include farmyard manure, slurry, sewage sludge, crop residues and green manures. Their quality and availability will vary from farm to farm and from year to year. They are nevertheless a valuable resource requiring careful management to ensure that they remain a benefit and do not become a pollutant.

Fertiliser choice



  • FYM
  • Solid: straight
  • Slurry
  • Solid: compounds
  • Sewage sludge
  • Liquid
  • Crop residues


  • Green manures



It is important to provide a safe storage capacity with protection against overspill. A farm waste management plan (see chapter 6) further helps to minimise this risk. The nutrient value of organic manures needs to be determined so as to make full allowance for them in the fertiliser plan.

...manufactured fertiliser

Manufactured fertiliser may be solid or liquid and the former group may be single or straight (i.e. supplying one major nutrient, usually nitrogen) or compound , a mixture of various nutrients in differing proportions designed to meet the typical needs of specific crops. Liquid fertilisers are transported, delivered and stored in bulk, whilst solids may be handled in bulk or in bags. The first step is to choose a good quality product.

Solid fertilisers vary hugely in their quality and the difficulties in handling inferior products may quickly outweigh any cost savings in their purchase. A good quality fertiliser needs to be free flowing and consist of even sized particles (ideally 2- 4 mm) that do not shatter in use. A measure of the quality is given in the spread pattern (SP) rating and products with a high rating should be chosen. 'SP5' is top grade. and handling

All fertilisers will deteriorate unless properly stored. Additionally, they are valuable commodities and some are a fire risk. Responsible storage, considering the needs of both operator and environmental safety, is important. Stores should be secure against unauthorised entry and both buildings and vehicles should carry the appropriate hazard signs for transport. Ammonium nitrate should not be stored with other combustible materials.

Storage of fertiliser

Detailed guidance on the storage of fertilisers is given in The Health and Safety Handbook published by the Fertiliser Manufacturers Association. key points are:

  • Stores should not be located near hospitals, schools etc. or potential sources of fire;
  • Fertiliser should not be stored where it can become affected by heat or mixed with combustible materials;
  • Urea and ammonium nitrate based fertilisers should be stored separately;
  • The top stacks or heaps should be at least 1m below eaves, beams and light fittings;
  • Bags should be stacked in stable heaps on level, well-drained ground.


Transporting ammonium nitrate

Most fertilisers are subject to national and international regulations with particular reference to the hazards of handling ammonium nitrate. There are some exceptions, but the general rules are:

  • All vehicles must be diesel powered;
  • Drivers of vehicles >3.5 tonnes must have special training;
  • Drivers must carry documents indicating the nature and danger of the goods, safety measures, first aid measures, fire-fighting requirements and measures to be taken in the case of deterioration of packages;
  • Vehicles must display an orange hazard warning triangle when loaded. It must be removed when the load is removed.
  • The regulations do not apply when no more than 10 tonnes are carried on an agricultural vehicle over no more than 12 km between land occupied by the same farmer.

Note: This is an outline of the Regulations which are covered in the Dangerous Goods: An Operators Guide published by the Freight Transport Association.


The DEFRA Code of Good Agricultural Practice for the Protection of Water (COGAPP) is an essential source of guidance not only on the storage, but also on the use of fertilisers. Managers and operators need to be familiar with these guidelines so that, for example, solid fertilisers are not stored close to field drains or watercourses, where leakage is likely to lead to prosecution and a fine.

...proper machinery operation

A fertiliser spreader is a precision tool and yet a survey has shown that over half of those in use in the UK are never calibrated and a similar proportion of operators have never seen the machine manual. One of the practical difficulties here is that there generally has to be a gross inaccuracy in application before an effect can be seen in the crop, and then it may be weeks, or even months, after the event. But damage does not have to be visible, and a uniformly green crop does not necessarily indicate optimum use of fertiliser.


Achieving a uniform spread pattern can be tricky and requires attention to detail. At the top of the list is training of the operator and regular maintenance of the machine. next comes calibration, and this should be done with all the products to be used. Settings for each product can be noted to reduce (but not eliminate) the setting-up procedure later. Spread patterns can be checked using trays, but the height recommended by the manufacturer for the required working width needs to be set in the field. This is because running in deeply rutted tramlines affects the working height above the crop.

...before starting

Weather conditions are important. Wet and windy conditions should be avoided as should applying to, or travelling on, waterlogged, frozen or snow-covered ground. Ringing the local weather centre can provide an accurate forecast for the ensuing twelve hours and is worth the effort if there is any doubt about whether to start a task. the field

During application the machines performance should be continually monitored. Spreading on headlands, water buffer zones and in hedge bottoms and ditches must be avoided and this can be helped by the use of devices on spreading equipment, such as deflectors, tilts and cut-offs, all of which assist in achieving accurate placement in these vulnerable areas.  In the body of the field, care is necessary to maintain the bout width, forward speed and flow rate. If necessary, consideration should be given to altering rates to meet special needs. If contractors are used for fertiliser spreading, the should be acquainted with the overall farm strategy and their work should be monitored to check that they comply.

The FACTS Scheme

Accurate determination of crop nutrient requirements is complex and needs to take account of a number of factors. Furthermore the ability of a crop to use up the available nitrogen depends on many factors, so requirements will not be the same from crop to crop nor from field to field. Good advice is vital.

The Fertilisers Advisors Certification and Training Scheme (FACTS) is designed to ensure the competence and professionalism of those giving advice on the use of fertilisers in the field. It is administered by BASIS, who also run a similar scheme for crop protection advisors. Advisors are required to undergo training followed by an examination before they are awarded a certificate of competence. Growers then have the assurance that advisors carrying a FACTS card have demonstrated that they have the experience and competence to give sound advice. If professional advice is needed - and , on crop nutrition matters, it usually is - always choose a FACTS qualified supplier or consultant.


The soil is a major natural resource for any horticultural or farming enterprise but it is frequently abused or neglected. Correct management is an essential element of ICM. Fertiliser choice, application and timing is one of the most vital inputs and yet is one of the least precisely controlled. A planned fertiliser strategy is economically and environmentally beneficial. Taking qualified professional advice and using good quality products will help to ensure that the right nutrients are applied in the right amounts in the right place at the right tome. Achievement of this will underpin all other ICM strategies.

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