A little while ago, a home vegetable gardener growing organically yearly applied poultry litter compost as his nitrogen (N) source. In the initial years, plant growth and product yield were excellent. In the fourth year, however, plant growth had slowed and there were visual signs of possible plant nutrient element deficiencies. In addition, product yield was also declining.

Wanting to know why these changes were occurring, the gardener asked his local agricultural county extension agent to come identify the plants' visual symptoms and help him determine their cause. Before making a judgment, the county agent collected a soil sample for submission to the state's soil testing laboratory.

The soil test results were given to me for interpretation and suggested corrective treatment. The soil phosphorus (P) level was 10 times that for sufficiency, while the ratio among the major plant nutrient element cations, potassium (K), calcium (Ca) and magnesium (Mg), was not in balance.

My comment to the county agent was to tell the gardener that there was nothing that could be done for dealing with the excessive soil P level, while the lack of balance among the cations could be corrected, but not easily. Therefore, my recommendation was to have the home gardener locate another garden site, and definitely no longer use the poultry litter compost.

What had happened? The poultry litter compost was not only high in nitrogen (N), the reason for its selection, but it was also high in P and K, far beyond plant requirements. With yearly application of this compost, the accumulation of both P and K in the soil was the reason for the poor plant growth and declining product yield.

What users of organic fertilizer materials must be aware of is that most of these materials are naturally occurring; therefore, they will contain various levels of most of the plant essential elements. Animal manures are usually high in the fertilizer elements, N, P and K, and composts that are derived from a mix of an animal manure and plant material will result in a concentration of these elements in the composting process.

Therefore, the user needs to know what is the elemental content of applied compost and the supplier should provide that information. In some instances, the fertilizer value of an organic fertilizer may be given, expressed as the percentage of N, P2O5 and K2O. However, it is not unusual that the levels given are as approximate values, whereas the user needs to know the exact values.

For some organic fertilizer products there are published elemental content values. Therefore, one might surmise that these published values are applicable to that particular material irrespective of its source.

Unfortunately, such values may not apply; therefore, the elemental content of the product being purchased needs to be known based on an assay of that particular batch. The range in elemental content may be considerable as was found by an elemental assay of five worm casting (vermicast) products, whose elemental content data were recently published in the May 2013 issue of Maximum Yield.

Such ranges in elemental concentration would affect the compost's application rate, and in turn when misapplied, lead to potential insufficiencies based on an assumed elemental content concentration, and not based on the actual elemental content of that particular batch.

As was noted in the experience described at the beginning of this article, the home gardener had not had his soil tested, an essential requirement to determine the initial fertility status of his soil. Then, by yearly testing, he could monitor the soil's fertility status, and in this case, that being affected by the poultry litter compost being yearly applied.

He could also observe what effect his cropping procedures were having on the soil's fertility status. The soil test would have revealed the buildup in P content and the developing imbalance occurring among the cations, K, Ca and Mg, so that corrective steps could have been taken, thereby saving his garden site.

When considering the selection of an organic fertilizer, find out what the major element content is as well as other companion elements. It may be that a companion element or elements will result in a plant nutrient element insufficiency, an insufficiency that is frequently not easily corrected after the fact.

As was learned by the home gardener mentioned at the beginning of this article, it was the companion elements P and K in the poultry litter compost that resulted in the loss of his garden site, as his selection of the poultry litter compost was made based only on its N content.

Before selecting an organic fertilizer, read the label and ask the provider of the product being considered to provide an actual elemental content assay for that material, not estimated content values. Inaccurate elemental content data can lead to misapplications, with the probable result being poor plant performance, or rendering the rooting medium infertile.

In my own personal experience, with the range of products available to a grower today, there is far greater danger in over fertilization and applying elements not needed as well as misapplication that can lead to plant nutrient element imbalances rather than insufficiencies due to a single element inadequacy.

Deficiencies are easy to correct, excesses and imbalances are not. Therefore, care in selecting an organic fertilizer product is essential by knowing its actual elemental contents, not its estimated or literature-derived values.