Seasonal Updates – Dr Chris Dowling
Plant tissue analysis has generally been the poor cousin of soil analysis when it comes to managing crop nutrition in broadacre crops. This is despite plant tissue testing having distinct superiority to soil testing in some situations and for some nutrients such as the trace elements. Long regarded as a vital nutrient management tool in many intensive crops, it can help fill many information gaps on broadacre crop nutrition. Broadacre cropping farming systems continue to evolve creating change in both the root zone distribution and availability of nutrients. Issues such as nutrient stratification (P and K), nutrient interaction with soil amendments and newly responsive nutrients are now key focuses of research. As a result, it can be important to look to plant feedback to understand in-season nutrient availability as a logical backup for soil testing, especially where nutrient management has changed and no soil test calibration is available.
At a glance
Plant tissue analysis is a direct measure of a plant’s ability to take up nutrients from the soil.
Plant tissue analysis can be a powerful tool for improving soil nutrient management.
Soil analysis predicts the quantity of nutrients available for uptake and plant tissue analysis determines if this uptake is realised.
Plant tissue analysis is more accurate than soil analysis as an indicator of the availability of nutrients such as calcium, magnesium and all trace elements
Regular plant tissue analysis, in conjunction with soil analysis, provides information that helps improve profitability and environmental care through targeted fertiliser applications.
Plant versus soil analysis
Plant tissue analysis is a direct measure of a plant’s ability to take up nutrients from the soil. This complements soil analysis perfectly because soil analysis predicts the quantity of nutrients available for uptake. Plant tissue analysis determines if this uptake is realised.
If soil and plant test results reveal different aspects about nutrient status in the soil and in the plant, the combination of the two tests helps to determine the reason. Common reasons for discrepancies include nutrient stratification through the soil profile and soil health problems such as acidity, alkalinity, salinity and sodicity, all of which can occur in the topsoil or subsoil. Root pruning by herbicides and soil-borne pathogens (such as nematodes) can also lead to discrepancies where plant tissue analysis indicates limited uptake from large reserves of nutrient measured in soil analysis.
In addition to being a direct measure of uptake of a wide range of nutrients, plant tissue analysis complements soil analysis in several ways. Plant tissue analysis is more accurate than soil analysis as an indicator of the availability of nutrients such as calcium, magnesium and of all trace elements. Interactions between nutrients (in the soil and for plant uptake) already have taken place when plants are analysed whereas they are predicted with soil analysis.
Ensuring plant samples are representative of the soils from which they are sampled is easier because subtle differences in plant growth can be seen at the time of sampling. This means plant tissue analysis is a better way of initially locating and logging (with global positioning system coordinates) long-term sample sites. In addition, it is easier to take comparative samples from areas of high and poor plant growth when plants are growing.
Although plant tissue analysis can point to soil problems such as acidity, salinity and sodicity, it is not as definitive in identifying these problems as soil analysis. The analysis also cannot be used to calculate the amount of lime or gypsum needed to rectify problems.
What to sample
There are preferred methods for sampling plant tissue that includes when to sample and what part of the plant to sample — these alter between areas and with crop or pasture type. While this can be confusing, it does not matter what is sampled provided the plant parts and sampling period are calibrated with yield and quality responses or are related to standard reference levels for healthy plant growth. Most plant tissue analysis services describe the plant parts they wish the grower to sample. For clovers and medics, the preferred samples are whole shoots (all above-ground parts) or young tissue. Depending on the farm location, young tissue could be the youngest open leaves, leaves with petioles (branches) still attached or for lucerne the growing tips.
Broadacre pasture grasses are usually sampled by taking whole shoots or young leaf blades. For broadacre cereal crops, the two main plant parts sampled are whole shoots and the youngest emerged blades. Whether whole shoots or youngest emerged blades are sampled depends on the grower’s purpose for the sampling and the interpretative criteria that exist for that area (see Table 1).
How to sample
When taking plant samples, it is important they are representative of the area and land management units from which they are sampled, are free of obvious non-nutritional limitations (such as waterlogging, low moisture stress, frost, extended low light) and contaminants (such as fertiliser dump sites and dusty roads or tracks).
Ensure equipment and hands used are free of contamination. Common contaminants include metals on hands after touching fencing, sunscreen creams, tobacco, and dirty scissors or secateurs used to cut plant material. Record the plant number or growth stage and send the sample to the laboratory quickly.
Interpreting plant tissue analysis
The concentrations of most nutrients reduce as plants grow because structural components make up a larger proportion of the plant. This reduced concentration needs to be considered when interpreting whole shoot samples (see Figure 1). There are several ways of accounting for this dilution. The method usually depends on how much information the grower can supply about plant growth stage and age. The most reliable method uses plant weight or a precise description of the growth stage at sampling. More sophisticated models include nutrient interactions (such as nitrogen x sulphur) and plant age effects.
A single critical value is inappropriate for whole shoots and while levels based on major growth stages are better, they are still not as accurate as calibrations using plant weight or related to each crop growth stage (Zadocks scale) for example. For cereals, it often perceived that it is easier to interpret youngest emerged blades, although nutrient interactions are more difficult to evaluate compared with analyses of whole shoots. If a nutrient imbalance exists, interpretations using the youngest emerged blade will be less accurate.
Act now to avoid future loss
If some parts of the crop or pasture are slower growing than others, yield or quality could be lost. Inadequate nutrient supply can impact on production and profitability without symptoms of nutrient deficiency appearing. Plant samples, even if they only compare poor and good areas, can alert growers to the need for nutrients. Even without visual differences in growth, plant tissue analysis used as a monitoring tool can guide growers toward yield and quality improvements in both current and future crops.
TABLE 1 Comparing some advantages and disadvantages associated with sampling cereal whole shoots (WS) and youngest emerged blades (YEBs)
|Youngest emerged blades
|Easy to sample.
|Too much plant material to sample and dispatch if sampling late in the season.
|Suitable assessment for nutrients that are immobile within the plant (such as manganese, boron, iron).
|Difficult to sample.
|Nutrient interactions within plants are easier to evaluate.
|Need the experience to interpret results accurately.
|Easier to sample if old plant parts are dead or diseased.
|Accurate interpretation is more difficult than it looks due to nutrient interactions and their variable mobility.
|More accurate assessment for nutrients that are mobile within plants (nitrogen phosphorus, potassium).
|Plant material of the same age is always sampled, so less affected by the ‘dilution’ effect in interpreting results. Critical ranges are more widely published.