All farmers are aware that soils are extremely variable from farm to farm. Soils however are made up of the same soil components in different proportions. The proportions of these components determine how soils behave physically (for example, moisture retention) and chemically (nutrient retention or fertility).
WHAT ARE SOILS
Soils are comprised of four basic units: sand, silt, clay and organic material (or organic matter). These particles are categorised, based on the size variation between the groups. Sand is the largest particle in soil.
Silt is next smallest in size, followed by the finest particle of clay. Organic material varies a great deal in size.
Effects of the various particle sizes on soil drainage properties are well-known.
The more large particles are present in the soil, the better drained is the soil. Conversely, when more fine particles, clays and silts are present, the poorer the drainage and therefore greater water-holding capacity of the soil. More organic material in a soil also increases moisture-holding capacity.
But what effect do these different types of particles have on how a soil holds nutrients?
HOW SOILS HOLD NUTRIENTS
In general, only two of the soil particle types, clay and organic material actually hold soil nutrients.
Clay and organic material are in fact charged particles. Scattered over the surface of these particles are positive and negative charges. This is due to their particular chemical properties. These charges attract nutrients of opposite charge to the surface of the clay (or organic material).
Negative charges attract nutrients such as Potassium, Calcium, Magnesium, Iron, Manganese, Zinc and Copper.
Positive charges attract Nitrogen, Phosphorous, Sulphur, Molybdenum and Boron.
How strongly the nutrients are held by the soil differs with the nutrient concerned. Nitrogen, for example, is leached relatively easily whereas other nutrients such as phosphorus and sulphur do not leach through the profile so easily.
Soils with more clay and organic material tend to hold both more water and more nutrients. When less clay is present in a soil, for example, a sandy loam, nutrients may tend to be washed (leached) through the soil profile by rainfall or irrigation.
Adding organic material (as in a cover crop prior to planting) tends to increase moisture and nutrient retention in sandy soils.
WHAT ABOUT 'RED' VOLCANIC SOILS
Coastal volcanic 'red' soils differ a little from other soil types in the way they hold nutrients.
These soils are red in colour due to the presence of high levels of iron and/or aluminium. Unfortunately, iron and aluminium oxides (known as sesquioxides) tend to 'fix' phosphorous and sulphur. 'Fixing' means a proportion of these elements are unavailable to plants.
Not all red soils have this property. So-called red sandy loams, for example, do not fix significant amounts of phosphorous or sulphur.
When soils do have this effect, more phosphate may need to be added, so that a proportion of what is added remains available to plants.
WHAT EFFECT DOES SOIL pH HAVE ON NUTRIENTS?
Most well-drained horticultural soils suffer from low pH. pH is in fact a measure of hydrogen ion concentration in the soil. As soils are leached (in well-drained soils), nutrients are washed out leaving negative-charged sites occupied by hydrogen ions (see below).
Increasing hydrogen ion concentration (low pH) also affects availability of nutrients.
Low pH (below 6) decreases availability of nitrogen, potassium, sulphur, phosphorous, calcium and magnesium. Micronutrients are, however, most affected by pH decreases, particularly molybdenum, copper and zinc.
Low pH also increases toxicity to plants in high concentrations (typically below pH 5). Coastal red volcanic soils tend to be naturally high in magnesium and aluminium.
Keeping soil pH in the 6 to 6.5 range keeps micronutrients (copper, zinc, boron and molybdenum) and calcium available to plants as well as reducing concentrations of toxic elements such as manganese and aluminium.
Having adequate quantities of all 12 nutrients in the soils is essential for optimum tree growth.
Tree crop yields often suffer prior to showing visual deficiency symptoms. A soil test is extremely useful to determine what nutrients need to be applied for best crop growth. The soil test also shows when pH adjustments are required and when toxic elements such as manganese and aluminium are in excess.
The only sure way to know what's going on in your soil is to get a soil test.
DATE: September 1988
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