The difficulty with soil analysis is that it cannot measure some of the most important factors that affect how a tree absorbs nutrients from the soil.
Temperature, soil wetness, compaction and drainage are just a few of these factors. This is the reason why we turn to tissue analysis.
It is one thing for a given amount of an element to be present in the soil and another to have a root system capable of picking it up.
Potassium fertilisers
Most fruit growers don’t seem to understand the effects of potassium fertilisers on their orchards.
They think that applying more potassium will improve both fruit colour and firmness. This is just not true.
Once potassium reaches a satisfactory level, about 1.5% level in midshoot leaves in mid-summer, applying more is simply a waste of money.
Not only is it a waste of money, but high levels of potassium lead to all sorts of problems such as magnesium deficiency. Even worse, high levels of potassium in the tree reduce its uptake of calcium.
In addition, applying potassium when it is not needed by the tree can reduce the uptake of phosphorus and hence affect tree growth and cropping.
Tissue analysis
Chemical analysis of leaves has become the standard diagnostic technique for determining the nutrient levels of horticultural plants since 1956.
Quotations:
“Soil chemical analysis generally is not regarded as being very useful for fruit trees”.
Childers, N.F. (1975) Modern Fruit Science 6th Edition (Hort. Publ. New Brunswick N.J.
“The main difficulty with soil analysis is how to obtain soil samples representative of the various zones of root nutrient absorption and how to weight the relative contributions of those zones. The main zone of feeder roots is generally at a depth of 15–45 cm, but roots can extend to a depth of over 10 m and can extract nutrients from lower zones should deficiencies develop in the main feeding zone.”
Leece, D.R. (1976) Diagnosis of nutritional disorders of fruit tree by leaf and soil analyses and biochemical indices.
“Bould, (1963) in a review, shows that soil tests for nitrogen have not been successful for predicting the nitrogen requirements of fruit crops owing to difficulty in measuring the great range of factors which control the rate of mineralisation of organic soil nitrogen. Greater success has been achieved in correlating soil phosphorous and plant response, but the picture is often confused by phosphorus reserves in the tree—which may be revealed by leaf analysis.
Thus, it would appear that if the question to be answered relates to the current nutritional status of the plant, then plant analysis is likely to be a more successful tool than soil analysis is.”
Leece, D.R. (1968) The concept of leaf analysis for fruit trees.
“If you want to know what nutrients to add to your orchard’s soils, don’t bother with the soil itself. Focus on the leaves.
If the grower gives me a copy of their soil analysis, I usually discard a lot of it, because it doesn’t have much value. It is not that soil analysis has no importance. In fact, it is crucial to take soil samples to analyse the soil before planting so that the land can be prepared.
It is just that’s the tree that sets the crop, not the soil.
Soil analysis can be useful for determining pH, lime or gypsum requirements, as well as changes in certain elements through soil layers, but it doesn’t tell you what the tree is using. In fact, if a grower isn’t careful, he can actually be misled by soil analysis, no matter how thorough it is. That’s because some types of nutrients may be present in the soil in sufficient quantities but can’t be used by the trees because of insolubility, competing nutrients, weakened root systems, drought, and a host of other causes.”
David Eddy. Look Up for Guidance—American/Western Fruit Grower (2005).
“Soil analysis can provide the basis for a basal fertiliser program. However, monitoring of the fertiliser program and the degree of optimum nutrition is best monitored using plant tissue analysis.”
Lee Coats. Agritech Labs Services. A guide to soil and plant tissue analysis. Good Fruit & Vegetables Sept. 1998 page 52.
“Soil analysis can help to point out possible problem areas (e.g. related to pH, salinity, excess lime or aluminium), but it is not easy to use predictively in woody perennials.
Soil testing has not been widely successful as a method for determining nutrient needs of tree crops because close relationships are not found reproducibly between soil test values and the nutritional status of trees due to the large amount of unexplained variability.
Plant analysis is the most useful technique both to confirm diagnoses of specific nutritional problems (deficiency or toxicity) and to monitor the success of fertiliser programs.
There is an extensive literature on plant analysis for a wide range of crops, and the concepts involved have been reviewed many times.”
J.B. Robinson Tree mineral nutrition. Acta Hort. 175, 1986.
“Soil testing provides a means of monitoring soil pH and estimating nutrient supply. However, a poor relationship exists between soil and plant nutrient levels in perennial crops.
Often fruit trees contain sufficient levels of a nutrient even though soil test values are low. Conversely, high soil nutrient levels do not assure an adequate supply to the tree.
Soil tests do give a reasonable estimate of nutrient status of shallow-rooted crops such as strawberries but not of fruit trees.”
Eric Hanson, Dept. Hort. Michigan State University. Fertilizing Fruit Crops.
“Because of the complexity of soils the availability of elements and the difficulty of relating nutritional status of the samples surface soil to the whole rooting volume of the tree, nutrition management based on soil analysis alone is not recommended.”
R.A. Stephenson et al. Assessment of nutritional status of perennial fruit crops. Actoa Hort. 175 1986.
“Soil sampling and testing in established orchards are used for troubleshooting or monitoring rather than routine fertiliser recommendations.
Routine soil testing every two to four years is recommended as a supplement to tissue testing and to monitor changes in orchard soil nutrient status.”
Tree Fruit Nutrition—Shortcourse Proceedings published by The Good Fruit Grower USA (1994); page 154.
See this article in Tree Fruit April 2016
