The value of measuring the texture of your soils
NRM
Agriculture
Understanding the texture of our soils helps with cultivation planning as well as water and nutrient management. Even where soils have been farmed for a good number of years and the characteristics of fields are well-known, measuring their soil texture will help finetune operations.
Decades ago, soil texture maps were produced in detail, but mapping required extensive manual coring. As soil texture influences the timing and type of cultivation required, full texture maps are preferable to average field figures. Today, for a small additional cost, laser analysis is undertaken by NRM to assess samples for their sand, silt and clay content alongside nutrient content analysis. These maps, along with yield maps, are valuable in identifying areas which could be used for environmental schemes.
Where do your fields fit in the soil texture triangle?
The textural class of a soil is determined by the balance of sand, silt and clay. This is summarised in the soil texture triangle below, which also provides the classification of heavy, medium, and light soils used in environmental schemes.
Ideal soil proportions by volume
Although here we are focussing on mineral content, only 45% of a ‘healthy’ soil should comprise of sand, silt and clay. The remaining 55% should comprise mostly of soil pores, ideally with half of the pores filled with water as opposed to air to allow a supply of nutrients to the plant.
Relative particle sizes
Nutrients are held on both the negatively charged clay particles and on organic matter by electrostatic forces. Clay content greatly influences the soil’s ability to hold the positively charged cations of calcium, potash, magnesium, and sodium, which is referred to as ‘Cation Exchange Capacity’ (CEC)—an ability that NRM can measure as part of our soil analysis services. CEC is expressed in meq/100 grams of soil (milliequivalents/100 grams).
Particle sizes are much larger in sand and silt and therefore have a much lower holding capacity. Sand therefore has little-to-no levels of CEC, and silt has very low levels. For an adequate nutrient holding capacity, soils need a CEC of 17 meq/100g or above. Holding capacity can be improved by increasing organic matter content, which is vital in sandy soil to hold nutrients for crop uptake. Phosphate is also held on the clay particles, first by adsorption and then absorption over time.
Comparison of particle sizes
Water holding capacity of different soils
With increased clay content, water holding capacity rises, although some may not be accessible by plant roots as it is held in the micropores.
Compaction
The risk of damage to soil structure by compaction is greatest in high clay content soils, resulting in significantly reduced root proliferation, which in turn leads to poor nutrient and water uptake. Although there are times when some damage has to be incurred to harvest crops in wet conditions, this can be minimised by the use of appropriate low ground pressure tyres and reduced axle weights. However, this damage should be rectified at the earliest opportunity to optimise soil density for effective crop production. Soil texture maps will aid harvest planning and help farmers take informed remedial action as soon as conditions allow.
Remember: sand, silt and clay are the building blocks which together form the structure of soils. As the soil texture we have will not change, it has a major impact on all farming systems and planning decisions. Discuss with your agronomist or contact NRM to monitor your soil texture more closely to help improve the sustainability and profitability of your farm.
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