Home About Us Our Services Publications Testing Submissions Contact Us Links

Interpreting a plant analysis

Interpreting a plant analysis is not easy. Although the association between nutrient uptake and plant growth is well studied, we still don’t have enough data for a number of crops, particularly at the seedling stage, and for concentrations at or near toxic levels.

Figure 1 shows a typical relationship between the nutrient concentration in a plant and yield. The steep slope in the deficient zone shows that the range in concentration between deficiency (with visible symptoms) and the critical concentration (no visible symptoms) is small. For some elements and some plants, the techniques needed to detect these small differences have not yet been developed.

Figure 1. Idealised relationship between nutrient content of a plant and yield.

Critical values

A critical value is the concentration below which deficiency occurs. But without a corresponding upper value, it is not very helpful.

A standard value is determined from the analysis of large numbers of samples collected from normal crops. Being a single value, it too offers limited usefulness.

Ranges of concentrations give values classified as, for example, low, adequate and high. The effects of time of sampling, cultivar, soil moisture, temperature and light can significantly affect the relationship between nutrient concentration and plant response. Therefore, even a defined sufficiency range may not apply to all situations or environments.

Tissue concentrations can be affected by differences in plant growth. Under normal conditions, nutrient uptake determines plant growth in a predictable way during most of the crop’s growth. However, exceptional conditions can cause nutrient accumulation or dilution. Thus, it is essential that the time of sampling, stage of growth and crop history be taken into account when a plant analysis is interpreted.

Even plants within the same species will vary in their ability to take up nutrients. The difference is genetic in origin. But much research is still needed on the effect of genotype on plant nutrient contents.

Nutrient interactions

Interactions or the balance between nutrients within a plant are critical. Varying one nutrient from deficiency to excess can alter the concentrations of other nutrients.

An appropriate interpretation of a plant analysis can be done by comparing the nutrient concentration against a sufficiency range: whether it is less than, greater than or within the sufficiency range. Soil test data and cultural practice information can help explain insufficient or excess concentrations.

Causes of deficiency or excess

Nutrient concentrations can fall outside the sufficiency range for many reasons: low or high soil nutrient levels, low or high soil water pH, improper fertiliser application, soil compaction, nematodes and weather. In most cases, the nutrient concentration found in plant tissue matches the soil level or pH better than the amount of fertiliser applied. The use of a balanced, long-term lime and fertiliser program will give better results than any one lime or fertiliser treatment.

Nutrients

Soil P and plant P are closely related. P uptake can be affected by cool soil temperatures, waterlogging and very low soil pH.

Soil K and plant K are also closely related.

Soil Ca and plant Ca are usually positively related, but are affected by fertiliser, weather and soil pH: a higher soil pH reduces the association. Heavy applications of N and K fertiliser will tend to decrease the uptake of Ca.

Plant Mg uptake can be reduced by a decreasing soil pH (below 5.4) and an increasing soil K or Ca level. A Mg deficiency can be partially corrected just by liming. This is of primary importance with forages where a high Ca:Mg ratio can promote grass tetany. In this case, the balance between Mg and both K and Ca must be managed. As with Ca, a higher soil pH reduces the association between soil Mg and plant Mg.

In general, as soil pH increases, the availability and, therefore, uptake of Cu, Fe, Mn, and Zn decreases. An increasing soil organic matter content intensifies this effect. The primary exception is Mo, the availability of which increases with increasing soil pH.

B deficiency is due primarily to insufficient B in the soil. The corrective treatment is to apply B fertiliser can correct it. B toxicity can result from overfertilisation.

Cu deficiency occurs primarily on soils with a high organic matter content and possibly on sandy soils with pH values approaching 7.0. Cu toxicity could occur with the long-term application of large quantities of some animal manures, particularly poultry manure.

Many soil and plant factors can influence the Fe level in plants. Deficiency may occur when the soil-water pH is near neutral and the soil is high in organic matter.

Soil-water pH exerts a very strong influence on Mn availability in most soils: a pH of <5.4 can lead to toxicity, and a pH of >6.3 can lead to deficiency.

Plant analysis has trouble detecting Mo deficiency, particularly in legumes, whose symbiotic root bacteria require higher levels than the plant does. The normal treatment is to sow seed dusted with Mo. pH has an effect too: most legumes respond best at low soil pH (5.2), and respond less well as the pH increases. Therefore, maintenance of the proper soil pH will help eliminate Mo deficiency.

Zn availability is related to both soil pH and soil Zn level. Soil Zn is usually a good indicator of Zn availability, although Zn uptake normally decreases as the soil pH increases. A Zn deficiency can be readily corrected by applying Zn fertiliser.

Al is not a plant nutrient but can affect plant growth. High Al levels in the plant are usually the result of either a very low soil pH (<4.8) or anaerobic soil conditions. As Al does not readily enter the plant, its presence in the plant in high concentrations indicates extreme soil conditions.

Accurate interpretation

This article shows that the interpretation of a plant analysis and an appropriate corrective recommendation can be a complex task requiring skill and sufficient knowledge of the site conditions. Details of the crop, site, weather and site history are essential to an accurate interpretation of the results and appropriate corrective treatments. Without them, accurate evaluation of a plant analysis result is impossible.