Fertiliser-Soil-Plant Interactions

These interactions are exceedingly complex and in part biochemical. They are not fully understood and much work remains to be done in this field - the huge variety of plants, soils and climates further lengthens the task. Phosphorus is of special importance for root development and in the ripening of seeds and fruit. 

The addition of phosphatic fertilisers to soils can greatly increase crop yields although the concentration of P in the crop itself is usually only slightly increased. 

Added fertiliser rarely travels more than a few centimetres before reaction with the soil components. Subsequent loss of P is mainly through crop removal and sometimes soil erosion. Drainage losses are usually relatively low. 

Soils may contain 0.02 0.5% P and some typical values are indicated in Table 12.6. Only a small fraction of the P present in the soil is usually available for immediate plant nutrition. 

Acid soils favour the absorption of H2PO4 by plants and this ion is absorbed much faster than HPOy. At soil pH 5.0-7.2, H2PO4 dominates whereas at pH 7.2-9.0, HPOy is mostly present. Maximum uptake of P from the soil usually occurs at about pH 5.5-6.5 and a minimum uptake at pH 7.5-8.0. A high uptake of N from fertilisers occurs over a wider range of pH 5.5-8.0. Much of the absorbed phosphate is converted by enzyme processes to the many organophosphate esters which are present in plant cells. A large part of the organic phosphorus content of soils is provided by bacteria and their dead residues. The total amount of P in soils does not necessarily relate directly to the availability of the element to plants. 

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All chemical fertilisers operate well below 100% efficiency and a proportion of the N, P and K is always lost and does not appear in the crop. The required concentration of available nitrogen is more easily lost from the soil than that of the other two elements, which can to some extent be stored. Phosphorus is usually considered to be the least mobile of the three fertiliser elements. The chemistry of fertiliser-soil-plant interactions is generally very complex and remains far from being completely understood. 

D.E. Kissel, D.H. Sander, and R. EUis, Fertiliser-Plant Interactions in Alkaline Soils, Chapter 5 in (9). 

It is generally believed that the plant obtains its phosphate ions not directly from the applied fertiliser but from the reaction products of soil and fertiliser. Interaction between the latter is very complex and many variable factors such as aeration, temperature, humidity and pH are involved. Under some circumstances the rate of phosphate uptake from the soil can be increased 10-fold by changing pH from 8.7 to 4.0. 

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