Soil analysis: key to nutrient management planning

Frequency of sampling

Soil nutrient levels do not alter markedly over short periods of time unless major factors of supply or demand are introduced. There is therefore no point in incurring the additional cost of sampling more frequently than necessary, except perhaps as a check to ensure that earlier sampling has been carried out properly. Additional sampling may be justified when there is a major change to husbandry practice - for instance alteration of cropping or manure policy. General guidelines for sampling frequency are as follows:-

			Sample every
		Permanent grass	7 years
		Intensively used grassland	3-4 years
		General arable cropping	4-5 years
		Arable/grass systems	4-5 years
		Field vegetables & horticulture	2-3 years

Analysis

There are a number of reasons why different results may be obtained for identical samples analysed by different laboratories. The most obvious reason is that different extractants may have been used. Variations in results can also occur if identical protocols are not followed - techniques of soil drying, grinding and sieving, reagent and equipment temperature, extractant concentrations, extraction shaking, stirring and filtration, and extraction time can all affect the result. Results are usually reported in mg/l but where a laboratory measures soil by weight instead of volume, the units will be as mg/kg. This can give rise to differences on some soils - particularly those with high levels of organic matter.

If different laboratories are used for analyses these factors must be taken into account.

Interpretation of soil pH

In England & Wales target pH (measured in water) is 6.5 for arable crops and 6.0 for grassland. Optimum range in Scotland where Scottish Agricultural College (SAC) measure pH in calcium chloride is 6.0 - 6.5 for arable crops and 5.7 - 6.2 for grassland.

In-field measurements using pH indicator on some soils where free chalk or lime particles exist may give lower values than laboratory results for the same field. This is because grinding the soil for laboratory analysis pulverises any chalk/lime particles and the pH as measured is increased.

Acidity below pH 6.0 will reduce the availability of nutrients, especially P.

Soil analysis may overstate the availability of K on some alkaline soils where pH is over 7.0 and responses to added potash may occur at higher soil K levels than would be expected.

Availability of trace elements is radically affected by pH and the need for trace elements should only be assessed after any required amendment of acidity has been undertaken and has had time to take effect.

Influence of soil pH on plant nutrient availability