Phosphorus, Potassium and Sulphur Cycles in Agricultural Soils

Keywords: Phosphorus, Potassium, Sulphur, Cycles, Soil fertility, Soil nutrient reserves, Crop nutrition, Fertilisers, Manures, Fertiliser recommendations, Environment.

This paper discusses phosphorus (P), potassium (K) and sulphur (S) cycles in agricultural soils in relation to the essential role each of these three nutrients plays in food production. Much of the discussion deals with each nutrient separately. The final sections, however, consider for all three nutrients the appropriateness of soil analysis for assessing soil nutrient status and the factors that can effect the interpretation of the data relative to fertiliser recommendations and the possible environmental consequences of losses of these nutrients from the soil compartment of the cycle. These nutrients are considered separately because although there are similarities in their behaviour, there are important differences in part related to the fact K exists as a cation, P and S as anions. All three nutrients are taken up by plant roots from the soil solution as ions, H₂PO₄^–, HPO₄^2- and K+ and SO₄^2-, respectively. The chemistry of soil P, however, is more complex than that of K. Almost all the K is present as the monovalent K⁺ ion while P and S are present both as ions and as inorganic and organic molecules. To fulfil their essential role in crop nutrition these three nutrients are required in quite large amounts, K more than P and S which many crops require in about equal amounts. For annual crops the daily rate of uptake, in kg/ha, is greatest in the early stages of growth. Therefore, the readily available pool of nutrients has to be sufficiently large to rapidly replenish them in the soil solution, for P sometimes as much as ten times each day, during the period of maximum daily uptake. For some vegetable crops which have relatively slow root growth, often combined with a high nutrient demand from the plant shoot, satisfactory early growth and yield often requires placing starter fertiliser near seeds or transplants even on fertile horticultural soils receiving optimum rates of broadcast fertiliser. For many annual crops the cumulative uptake of most nutrients with time is small initially then increases rapidly then decreases again and total uptake is maximum near harvest.

Both P and K can accumulate as plant available reserves in many soils when the nutrient balance (nutrient applied minus nutrient removed) is positive and both can be released from such reserves when the nutrient balance is negative. The immediate plant availability of these reserves depends on the rate at which they are transferred to the soil solution from the various soil pools in which they are held. In this paper we use Olsen P and exchangeable K (K_ex) as a measure of the readily available reserves of P and K, respectively. For S, as for N, plant analysis appears to offer more promise than soil analysis to determine S sufficiency or deficiency. This is because sulphate ions are leached from the soil in through drainage and reserves usually accumulate in soil organic matter that must be mineralised to release plant available sulphate.

A E Johnston and P R Poulton, IACR-Rothamsted, Harpenden, UK.

J K Syers, University of Naresuan, Phitsanulok, Thailand

44 pages, 12 figures, 14 tables, 83 refs.