Soil Chemical Analysis

Soil samples, collected from the field, are extracted with a solution of weak acid, salt, chelating agent, or a combination of these compounds to determine the amount of a given nutrient, or its constant portion thereof, that will be released by the soil for plant uptake during the entire growing season. Results of such tests are expressed in kilograms per hectare or pounds per acre and are assigned a nutrient availability designator, e.g., low, medium, high, or very high.

Determining soil reaction or pH is the first, most important step in soil analysis, which then indicates the need for other tests. For example, a pH below 6.5 (acid soil) indicates that a lime requirement test should be performed, whereas pH above 7.0 (alkaline soil) indicates the need for salinity and exchangeable sodium tests. Soils are routinely tested for available K, P, and Mg, and less often for available B, Cu, Fe, Mn, and Zn.

Soil chemical tests have three major limitations. The first is the difficulty of obtaining a representative soil sample. This is due to high natural soil variability and the uncertainty of how many and where soil samples should be collected within the volume occupied by tree roots. The spatial distribution of tree roots and thus the extraction of nutrients from the soil profile are not uniform. There is, however, a lack of quantitative data on the pattern of nutrient extraction by the roots of fruit trees from different soil depths. Without this information, it is uncertain how sampling locations should be spatially distributed within the soil profile and how to properly interpret the re sults of soil tests conducted on samples collected from different soil depths. The second limitation reflects the fact that no chemical extracting procedure adequately mimics the natural process of nutrient release by the soil. The latter is affected by constantly changing biotic and abiotic conditions in the soil, i.e., the factors totally ignored by soil chemical tests. The third major limitation is the lack of sufficient data to correlate the results of soil tests with fruit tree responses in field fertilizer trials. It is well known that such responses may be modified by various production systems, rootstocks, and even fruit tree cultivars, thus further complicating the task of properly interpreting the results of soil tests.

Increasing sampling intensity, enhancing the knowledge of how the results of soil tests correlate with tree responses in field fertilizer trials, and the availability of trained personnel to interpret soil tests will mitigate the limitations discussed previously. Nevertheless, at the current stage of knowledge, the results of soil tests are only used to guide preplant fertilizer applications and in existing orchards to supplement the information obtained from tissue chemical analyses. Additionally, soil chemical tests are indispensable for properly managing extreme soil environments such as acidity, alkalinity, salinity, sodicity, and B toxicity.

Plant Tissue Analysis

Plant analysis is a widely accepted method of estimating fertilizer needs in deciduous orchards. Leaf chemical analysis is particularly useful in determining a fruit tree's nutritional status.

The results of leaf analysis are compared to leaf standards that designate sufficiency ranges of various nutrients. For the method to be reliable, sampling and handling of leaves must adhere to a prescribed protocol, which designates timing of collection, sampling pattern in an orchard, number of leaves per sample, size of a tree block represented by one sample, type of shoots from which the leaves should be sampled, and leaf washing, drying, and grinding procedures. Although the protocol may differ among various fruit-growing regions, generally, leaves (including petioles) are collected from the middle portion of current year, terminal shoots about 60 to 70 days after petal fall, i.e., from mid-July to mid-August for most of the fruit species grown in the Northern Hemisphere.

The concentration of a given element in a leaf integrates a number of factors that are known to affect the mineral nutrient status of the plant. These factors include, but are not limited to, soil nutrient availability, soil temperature and moisture, soil compaction and aeration, climatic conditions, plant genetics, rootstock, cultural and soil management practices, tree productivity and age, mechanical injuries, and damages caused by disease and arthropod organisms and nema-todes. The multitude of these factors makes the interpretation of the results of leaf analysis difficult and requires the involvement of well-trained personnel.

The results of leaf analysis provide no information on causal factors that led to the development of a particular nutritional status. Also, they reflect past conditions that may or may not occur in the future. Frequent analyses, conducted every year or two, may overcome this limitation by revealing possible trends in nutritional status that can be related to culture, environment, soil, and other variables.

In some countries, preharvest analyses of apple fruitlets or post-harvest analyses of apples are conducted to predict storage potential. These analyses usually include elements such as N, P, K, Ca, and Mg. The length of storage of a given lot of fruit is determined after comparing the results of analyses to specially developed standards for different apple cultivars.

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