Chapter 5. Soil Nutrient Supply


There are several ways to determine the nutrient supply in the soil:

A survey shows that many soils already have a sufficient supply of phosphorus and potassium.

Methods Of Evaluating The Soil

One way to determine the fertilizer requirement is to add to the soil whatever was removed in the previous harvest. This is a common approach taken by many gardening guides and fertilizer dealers, even those who use soil tests, and it often succeeds. But this method of adding what was removed is expensive, because it encourages the use of more fertilizer than necessary in order to hedge against uncertainties. It ignores the ability of the soil to bring about the release of nutrients from its reservoir of minerals. And it ignores the dangers of excesses and imbalances1.

An improvement is to have the soil tested. Tests done at intervals will determine whether or not nutrients are accumulating. A soil test done only once is still useful in determining what kind of fertilizer is necessary.

Soil tests have several drawbacks, owing to the approximations that are necessary to carry them out quickly and cheaply. Even so, they are often good in establishing the nutrient status of the soil.

Anyone obtaining a soil test should distinguish between the test results and interpretation on the one hand and the recommendations on the other. Some laboratories recommend a maintenance fertilizer application even though the tests show no fertilizer is necessary. Some laboratories recommend fertilizer even though they do not test for nitrogen or estimate the nitrogen release. When in doubt about the basis of a fertilizer recommendation, one should question the laboratory about its policy.

Another type of test is a plant tissue test. Tissue tests bypass some of the problems of a soil test by revealing directly whether a nutrient is deficient in the plant. A tissue test, however, does not indicate the cause of the deficiency. If nitrogen is deficient, is it because the soil has none or because bad weather or insect damage limited root growth? Is phosphorus low because the soil has none or because the pH is too low or too high? If one already suspects why and wants confirmation, a tissue test is excellent.

Soil tests and tissue tests are not alternatives, but complements, and the ideal arrangement is to use both. In an orchard, where roots descend below the level at which soil is tested, tissue tests are good for revealing deficiencies, especially in trace elements. A soil test will show the state of the topsoil, which is a major influence in determining the nutrients which eventually reach the roots. A soil test is particularly helpful if a cover crop is grown between the trees.

A soil test kit is an alternative to a professional soil test. The results are less accurate, but the tests are cheaper and much more convenient. One useful test, however, that kits do not offer is one for organic matter.

The major disadvantage of a kit is that it is weakest in the most important part of testing, the interpretation of the results. This can be overcome, however, by testing frequently and keeping records of test results, fertilizer use, the weather and the harvest.

If the tests and records are done carefully, the experience is invaluable, and eventually the results can be better than professional tests. This is so because professional tests cannot take into account all of the variations in soils and weather conditions that may exist from one area to another. Good records and on-site experience can more than compensate for the lower accuracy of a soil test kit.

Despite these tools of modern agriculture, some people are intransigent in their refusal to test their soil or to routinely dump fertilizers for maintenance. As it turns out, many of them do very well, and it must be admitted that soil and tissue tests are not necessary in determining what the soil can supply. What is necessary? I don't know, because I am not one of those who can confidently get along without a test. Chances are, however, that some useful practices are:

An aid to all of these methods for assessing the soil capability is a knowledge of deficiency symptoms. Though helpful, signs of a nutrient defiency are ambiguous, partly for the same reasons that a tissue test reveals the deficiency but not the cause. Often a plant will show signs of a phosphorus deficiency because the soil is cold; as the soil warms up, the symptom disappears.

Also, many nutrient deficiency signs look similar; it is often difficult to distinguish a nitrogen deficiency from a shortage of sulfur or iron; magnesium, manganese and copper deficiencies often have similar manifestations. Furthermore, water stress or a disease could produce the same symptoms as a nutrient deficiency.

A number of references are available, however, for those who want to develop their sense of plant deficiency symptoms [22], [25], [82], [32], [39].

Additional information on possible soil problems is available by looking not only at the leaves and top growth of the plant, but by carefully digging up a plant, gently removing the soil, and examining the roots for vigor and signs of disease or pest damage.

Roots growing in a fertile soil are more branched than in a poor soil, and they have a profusion of root hairs. Root hairs are fragile; plants must be gathered carefully to avoid losing them. If the roots are growing laterally and are long and stringy, with few hairs, they are searching for nutrients; if they are also long and seem to be searching for something but are growing vertically, they need water. If they are growing only near the surface the soil is too wet. If they are stunted they may lack oxygen; the soil may be too wet or compact. If they are thick and short, they may be suffering from a toxic element, perhaps aluminum or excess manganese in an acid soil, or the soil may have a high salt content.

Inspect legume roots for healthy, nitrogen-fixing nodules. These are round growths attached to the roots. Their size varies with the plant variety and is no indication of plant health; the number of nodules, however, is useful. A large number of nodules indicates that the plant is supplying a large amount of carbohydrates. Usually this is a sign of a healthy, vigorous plant, but it could also mean that the plant is missing an important trace element, such as molbydenum or iron, for metabolizing nitrogen. The nodules should be distributed throughout the roots; if concentrated only near the crown, especially in a heavy soil, most likely the nitrogen-fixing bacteria were unable to move through the soil and onto the roots.

Splitting the nodules should reveal a pinkish color in the interior. it indicates the presence of an essential enzyme-molybdenum-iron combination and is a sign of a healthy, viable nodule. It would confirm a sufficient supply of molybdenum and iron.

Another clue to a possible soil imbalance is to note any groups of plants that seem to be growing poorly as a whole. In most soils, for example, poor growth of grasses, grains and sometimes beets may be due to a nitrogen deficiency. A shortage of potassium often leads to poor growth of legumes and potatoes. Phosphorus and potassium are dominant requirements for most root crops.

On the other hand, weather variations may be of critical importance and could result in poor growth of one variety in a group without affecting the others. Peas, for example, growing in unusually cool and damp weather with too much nitrogen might be disappointing, because the onset of flowers may be delayed into the hot weather, when growth stops.

A Survey Of Soils

Figure Figure 1. Soil Test Results contains the results of a survey of about 1800 soils tested by Woods End Laboratory. More than half are from the northeast; about a third from the west and southwest; most of the remainder from the upper midwest, and some from the southeast. The availability of nitrogen was estimated from the organic content. The Bray P1 test estimates available phosphorus; it does not indicate the amount of available phosphorus but only whether the amount actually present is satisfactory. Usually a P1 value of about 40 lb/A infers that the soil has sufficient phosphorus for crops with a low requirement, and 70 lb/A for crops with a high requirement, although this may vary somewhat, depending on other soil characteristics.

Figure 1. Soil Test Results
Available Potash in 2000 soils Available Phosphate in 2000 soils Available Nitrogen in 2000 soils

Figure Figure 1. Soil Test Results infers that many soils have more than enough phosphorus and potassium for good plant growth and at least a third to half of the necessary nitrogen for a single crop. High phosphorus values, especially in the Northeast, are often due to a previous history of continual fertilization. Low phosphorus values are typical of many semi-arid soils, soils with a high pH, and many forest soils. Soils with low potassium are usually highly leached or from played-out hayfields.

The soils illustrated here are not representative of agricultural soils but only of those tested by Woods End Laboratory. Most were personal and market gardens managed organically.

1 One common example of the effect of excess fertilizer in the eastern states is a magnesium deficiency brought about by overuse of potassium fertilizer and/or low-magnesium limestone. For more details on the dangers of excesses, see table 2. Effects Of A Nutrient Disorder On Crop Quality and the discussions of nutrients in part III.    [return to text]

© 2013 Robert Parnes

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