ARCHIVE: Fertiliser Manual (RB209)

Important soil properties 

Soil texture

Knowledge of the soil type in the surface and subsoil of each field is essential for making accurate decisions on fertiliser and lime use. Without this knowledge it is not possible to use the recommendations in this Manual effectively and to achieve optimum benefit from them. Time is well spent therefore in acquiring and retaining this information because it is an intrinsic property of the soil that does not change with time. Soil type as used in this Manual is related to soil texture, which ranges from sands to clays. Soil texture is defined by the proportion of
sand, silt and clay sized mineral particles in the soil and can be determined in a number of ways:

  • Laboratory analysis of the proportion of the different mineral particles in the soil, followed by classification using the texture triangular diagram given in Appendix D of Controlling Soil Erosion (MAFF PB4093). 
  • Identification of the Soil Series for each field from the Regional Soil Maps for England and Wales, with classification from the accompanying Brown Book (available from the National Soil Resources Institute at Cranfield University).
  • Assessment of texture class by hand using the method given in Appendix 1, followed by classification. Guidance also is given in thinksoils from the Environment Agency which helps farmers and farm advisers assess soil and recognise problems of erosion and runoff. Included is a pocket-sized quick guide to soil assessment to help the identification of soil texture.

Soil structure

To achieve optimum economic yields, crops have to acquire sufficient nutrients and water from the soil via the roots. It is therefore important to develop and maintain a good soil structure so that root growth is not adversely affected by poor physical soil conditions, such as compaction.

Soil mineral particles can be aggregated together and stabilised, either by clay or organic matter, to form crumbs. Within and between these crumbs are pores (voids) that can be occupied by air or water, both of which are required for roots to function properly. If the diameter of the pores is too small root tips cannot enter and roots cannot grow to find water and nutrients. If pores are too large water drains rapidly from the soil and roots will not grow because the soil contains too little moisture. The aggregation of soil mineral particles defines soil structure. For example, sands are often without any recognised structure while loamy soils can have an excellent structure. Developing and maintaining a good soil structure depends greatly on good soil management including cultivation at appropriate times and depths, and minimising traffic over the soil when it is too wet. Protecting Our Soil, Water and Air: A Code of Good Agricultural Practice contains general guidance on practices that will increase and maintain the ability of the soil to support plant growth.

Soil organic matter

Soil organic matter helps bind soil mineral particles of sand, silt and clay into crumbs. It has a number of important functions in crop nutrition. It improves soil structure enabling roots to grow more easily throughout the soil to find nutrients. It holds phosphorus and potassium ions (the forms taken up by roots) very weakly so that they are readily available for uptake by roots. It holds a store of organic forms of nitrogen, phosphate and sulphur from which available forms of these nutrients are released by microbial action.

The amount of organic matter in a soil depends on the farming system, the soil type and climate. The interplay between the first two factors is such that, in general, for the same farming system, a clay soil holds more organic matter than a sandy soil, and for the same soil type, a grassland soil holds more organic matter than an arable soil. It is difficult to define a critical level of soil organic matter because there are so many combinations of soil type and farming system. However, maintaining and where possible increasing soil organic matter should be a priority.

Soil mineral matter

The types of mineral matter in a soil affect the reserves of plant nutrients and their availability. For example, a soil containing minerals from igneous sources is likely to contain appreciable amounts of potash and possibly iron whereas a soil containing chalk will contain appreciable amounts of calcium. Soils overlying chalk are typically alkaline (pH above 7) while those overlying sandstone are typically acidic.

Stone content and rooting depth

A large content of impermeable stones increases the speed of water movement through the soil and because there is less fine soil to hold nutrients, the availability of water and nutrients is lower than in largely stone-free soils.

Soil rooting depth is important because many crops have root systems with a potential to grow to a metre deep or more. In deep friable soils where roots can grow to depth they can take up water and nutrients leached from the surface soil. Shallow soils over hard rock and compacted soil layers limit root growth, restricting nutrient and water availability and hence crop yields.

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