Orchard nutrition 2: Soil and leaf analysis

Chemical analyses of soil and leaf samples give an objective guide that helps us to choose the most appropriate orchard fertiliser. If a fertiliser program is based on analytical results rather than on routine applications of NPK fertiliser every year irrespectively of the trees' needs, then it is likely that money can be saved and the quality of fruit will improve. For example, a survey of 20 apple growers from southern Victoria showed that growers often used too much nitrogen, to the detriment of fruit quality.

Data from analysis of soil is not used for the same purposes as that from leaf analysis. Soil analysis is best used before planting. The soil pH, its salt content, and the phosphorus and potash content are more easily manipulated before planting than they are once the block is planted. In planted blocks, soil analysis still gives useful background information, especially if salt problems or problems associated with low or high pH are suspected.

Once the trees are established, leaf analysis gives a better guide to the tree's nutrient status than soil analysis does because it shows what is happening in the tree itself. Soil analysis only shows what is happening to the reservoir of nutrients the tree has to draw on.

Sampling the soil

Soil samples for chemical analysis should be taken from at least 30-40 different spots in an orchard block and mixed together in one container. This should be done because soil fertility in an orchard block can vary as a result of previous lime and fertiliser applications. It is useful to sample two depths, for example, 0-150 mm and 150-300 mm, to indicate how far nutrients have penetrated and also to find how the fertility of the soil changes with depth.

The State Chemistry Laboratory makes six determinations on soil samples. These are generally all that are needed.

Description of soil

This provides a visual identification of soil colour and texture (sand, loam, clay loam, clay). These factors can affect lime and fertiliser recommendations.

Soil acidity (pH)

This will indicate how much lime is needed. Fruit trees can grow well even on rather acid soils, but if acidity drops below pH 5.5 valuable soil nutrients become less available and may be lost. Lime will help the growth of grasses and clover and hence preserve the content of organic matter in your soil. Sufficient lime should be added to bring the pH to above 6.0. Various types of lime are available. Dolomitic lime (which contains magnesium) should not be applied unless it has been found that the soil is deficient in magnesium.

Electrical conductivity (EC)

This measures the total amount of soluble salts in the soil, both naturally occurring salts such as common salt and gypsum, and those derived from fertiliser. A conductivity of 0.22 deci-Siemens (ds) per metre is equal to 0.033% of Total Soluble Salts. Values above 0.3 ds/m may retard the growth of roots but other factors, such as soil type and the kind of salt present, must also be considered.

Sodium chloride (NaCI - common salt)

This is determined if EC values are high. More than 0.05% NaCl in sandy soils and more then 0.1% in clay soils could harm fruit trees; such values indicate that better drainage would improve the growth of trees and prevent the accumulation of salt in the soil.

Available potassium (K)

Potash fertiliser is unlikely to be needed if values are above 100 ppm in sandy soils or above 150 ppm in loams and clay loams, provided these levels are present throughout the top 300 mm of soil.

Available phosphorus (P)

Values above 20 ppm (30 ppm in clay soils) in the surface sample suggest that superphosphate is unlikely to be needed. But unless phosphorus values are also high in the subsurface soil, it would be useful to add superphosphate before planting young trees to supply them with a reserve of phosphate for the future.

Soil structural tests

Other tests, such as slaking, dispersion and extractable cations (calcium, potassium, sodium and magnesium) may be useful in particular situations. Cation analysis indicates existing or potential soil structure problems and can be used in determining how much gypsum should be added to aid root and water penetration.

Sampling the leaves

Time of sampling

Leaf samples should be collected during the most favourable period for the particular crop; this is usually mid-summer (Table 1).

Table 1. Standardised tissue sampled and the best period to sample different crops in Victoria

* Mid-shoot leaf: a fully expanded leaf from the mid-portion of the current season’s terminal growth. Shoots selected should be of average length and vigour (not water shoots) and free from disease, insect or mechanical damage.

Sample collection

Obtain a bulk sample of 200 leaves from the block by sampling 20 trees in a systematic pattern (for example, by following two diagonals in an "X" pattern across the block). Take 10 leaves from each tree, from around the periphery of the tree at about shoulder height. Record those observations or information on orchard history that may aid the accurate interpretation of the analytical data - such as visual symptoms, crop load (heavy, light), tree vigour, soil management and fertiliser practices, irrigation, soil drainage and possible foliar spray contaminants. If a deficiency/toxicity is suspected, two blocks from nearby may be sampled; the suspect block and a "control" block of healthy trees of the same crop, cultivar and soil type. Comparison of the analytical results may confirm or deny the deficiency/ toxicity.

Transfer samples to the laboratory in polyethylene bags packed in crushed ice in a portable ice-chest, then store under refrigeration (0°C-4°C).

Interpretation of leaf analysis

Results of the chemical analyses are compared with standard values, so that a rating of the sampled orchard's nutritional status is obtained. Tables 2.1 - 2.6 give standard values for a range of fruit trees. The terms deficient, low, normal, high, and excess are defined as follows:

• Deficient: Deficiency symptoms are present; level is too low for best performance.
• Low: No visual symptoms; level is below normal and may be insufficient for best performance.
• Normal: No visual symptoms; level is normal and should be adequate for best performance.
• High: No visual symptoms; level is above normal and may be causing a nutrient imbalance.
• Excess: Toxicity symptoms may or may not be present; level is too high for best performance.

Orchard factors affecting the results of leaf analysis

Seasonal conditions and orchard management have important effects on the content of nutrient in leaves. These effects may be just as large as those caused by the addition of fertiliser. The following factors affect the content of nutrients in leaves:

Dry seasons- In dry seasons the concentration in leaves of almost all elements (except manganese, sodium and chloride) will be lower because of decreased uptake. In dry years, manganese, sodium and chloride levels may be higher than normal.

Weeds- Competition from weeds and grasses will lead to lower levels of nitrogen in leaves. Conversely, the use of herbicides around trees will increase the uptake of nitrogen and its concentration in leaves often more effectively than would nitrogen fertiliser. Irrigation also tends to increase nitrogen levels in leaves but decrease calcium.

Waterlogging- Waterlogging caused by poor drainage and/or excessive irrigation can cause many problems including loss of nitrogen because of denitrification. Waterlogging also reduces the uptake of calcium and potassium, but may increase the uptake of sodium.

Crop load- Heavy crops are associated with higher concentrations of nitrogen, calcium, and magnesium in leaves. Conversely, light crops are associated with lower concentrations of nitrogen, calcium and magnesium in leaves.

Variety/rootstock- Red Delicious apples have high levels of nitrogen and low levels of calcium compared with other varieties of apples. Dwarfing rootstock’s, such as M9 and MM106, result in the scion leaves having higher levels of calcium than trees on seedling rootstock.

Fungicides/foliar fertilisers- When fungicides or foliar fertilisers containing zinc and/ or manganese are used, the levels of these two elements will be high even after the leaves are washed. When copper fungicides are used during the season, they will raise the levels of copper in leaves.

Other elements- High levels of one element may result in low levels of another. A good example is the relationship between potassium, calcium and magnesium. Applying too much potash fertiliser may result in magnesium deficiency. Applying magnesium may result in bitter pit or poor performance in storage caused by low levels of calcium in the leaves and fruit.

Interpreting the results obtained during the first year

Because of the factors listed above, which can bias the results obtained from leaf analysis, caution should be used when interpreting the first results that come from a particular block. A general rule of thumb is that in the first year a change in the fertiliser program should only be made if the analytical value for a particular element is in the deficient or excess range of tables 2.1 - 2.6. If the value is only in the low or high category, it is worth monitoring nutrient levels during the following year rather than changing the fertiliser program in the current year. However, if the value is consistently (over two or more years) in the low or high range, remedial action is justified.

Summary

Soil and leaf analyses are valuable tools for the orchardist. The important points are to take the sample properly, store it adequately, and be aware of other factors that will affect the results (especially those factors that can be controlled through management, such as weeds and drainage).

Tables 2.1-2.6: Leaf compositional standards for stone fruit and pome fruit from Leece, D.R., Diagnosis of nutritional disorders of fruit trees by leaf and soil analysis and biochemical indices (1976). J. Aust. Inst. Agric. Sci. March pp 3-19.

Table 2.1 Leaf compositional standards for apples

Table 2.2 Leaf compositional standards for pears

"Table 2.3 Leaf compositional standards for peaches and nectarines

Table 2.4 Leaf compositional standards for plums

Table 2.5 Leaf compositional standards for cherries

Table 2.6 Leaf compositional standards for apricots