Phosphorus and potassium requirements of cereals

ABSTRACT

The aims of the review were to evaluate the present approaches to assessing the P and K requirements of cereals grown in the UK and to identify sectors where technical change or further research might benefit growers. The close links of P and K with N are discussed.

Growers in many areas of the UK have gradually built up the P status of their soils such that cereals tend not to respond significantly to fresh applications of P and, similarly, cereals on many soils do not respond to K. This general lack of response, confirmed by hundreds of field trials, led to the extensive use of a system of maintenance applications of both P and K designed simply to replace the nutrients which are removed in grain and straw. The main danger is that a system of maintenance dressings is introduced before soils become unresponsive to P and K, which can lead to a gradual run-down in fertility.

Some of the underlying chemistry of both P and K in soils is discussed with comments on both inorganic and organic forms of P and the nature of the main reserve of soil K, along with factors affecting its release. Also some background information is given on methods of soil analysis some of which are used routinely, while others are more suitable for research. It is argued that the main use of soil analysis is to identify soils which are most likely to respond to P and K and also to indicate where the nutrient status is high enough to suspend P and/or K applications for a while. An almost equally important role for soil analysis is to monitor changes in available nutrient status with time.

Once adequate reserves of P and K have been built up in soils there is considerable flexibility in the timing of fertiliser applications and this is discussed in the light of field experimental evidence. There are advantages in applying P and K to the most responsive crops grown in any rotation and basing long-term fertiliser applications on input/off-take balance sheets on a field by field basis. Growers on very sandy soils cannot expect to use a build-up and maintenance approach to K manuring which depends upon soils possessing adequate K buffering power, but they should rather plan to meet the needs of each crop in turn on such soils.

On soils of low P status, combine drilling of P fertiliser is still the most effective method of application. This may also be the case for K on some low K status soils. Estimates of available P and K as measured by routine methods of soil analysis change only slowly in the majority of soils even when no P and K are being applied. Hence on most well-managed fertilised soils it is not worthwhile having soil analysis for P and K done more often than every 4 or 5 years.

Removals of P and K in grain and straw depend mainly on yield and on the method of straw disposal. Studies in England & Wales and in Scotland have shown that, on average, barley straw, especially that of spring barley removes more potash by 2 or 3 kg k 0/t from soil than wheat straw. However, the potash contents of straws can be very variable.

Some of the inherent limitations of soil analysis, particularly when different soil types are involved, are discussed. Soil analysis should continue to provide the main means of identifying the need for p and K fertiliser but a more balanced mix of soil and plant analysis is recommended for diagnostic purposes where any uncertainties exist.

Some of the difficulties of interpreting plant analysis are discussed. Reasons are given for expressing the K concentrations in actively growing cereals on a plant tissue water basis rather than on a dry matter basis, which largely overcome the problems of interpretation caused by the natural maturing of the crop. Although viewed as extremely promising for diagnostic purposes, the K concentration in the tissue water of cereals does not yet on its own offer a quick and easy way of cross-checking on the K status of crops. This is partly because lack of K can be compensated by increasing calcium, magnesium and sodium uptakes. However, the main reason is because growers and their advisors need more basic information on the subject. It is recommended that information is sought as quickly as possible on the relationships between final grain yields and levels of K in plant tissue water between tillering and flowering.

In the longer term, studies on K levels in tissue water could be extended to include enhanced disease resistance, increased resistance to lodging and, particularly, the optimisation of water usage by cereals. It is interesting that some of the benefits of correcting K deficiency in cereals may arise from reducing calcium intake. A satisfactory understanding of these aspects of cereal nutrition will require more detailed studies on the distribution and function of potassium, sodium, calcium and magnesium in different types of plant cells.

At present there is little easily accessible data on the P concentrations in developing cereals grown in the UK. It is suggested that greater use of plant P analysis could resolve uncertainties where P status is borderline between sufficiency and deficiency and where a single set of Index ranges for the interpretation of soil analytical data may not be satisfactory for different sorts of soils. The relative merits of three different approaches to cereal plant P analysis should be examined in detail. These are, simple P concentrations expressed on a dry matter basis, P concentrations coupled with total N concentrations, possibly as products, and finally, a method of analysis involving the estimation of the inorganic P concentration in the developing cereal plant.

Even though cereal roots are efficient at taking up nitrate, more information is required on the beneficial influence of K in soil and subsoil on nitrate uptake. In fertile K-sufficient soils, the K seems to stimulate N uptake, probably in a cyclical manner, and provided the soils are P-sufficient, N uptake stimulates P uptake in amounts which are adequate for optimum growth. Caution is recommended when interpreting data on N and K interactions which have not been studied under field conditions.

Although there are well-known differences in the way in which different cereal species react to soil conditions, for many of the more general purposes of P and K nutrition all cereals except maize can be grouped together. In terms of detail, however, there are discernable differences such as spring barley requiring a higher P status than winter wheat.

This review, completed in February 1990 has 99 pages in the full document.