Interpreting the Report

The soil analysis report gives the levels of the various nutrients found in the soil sample. However, the numbers can be confusing. Different laboratories use different methods and will give different numbers on the same sample. They may even use the same methods, but express the results in different units. What should you do?

Forget the numbers. In addition to giving numbers, a good laboratory MUST give a description of the soil nutrients. Some will give a written description, saying for example “phosphorus is very low” or “calcium is high”. Others, such as Agro Services International give an easy to understand chart showing the status of each nutrient. Most people prefer the charts because they gives a good overall “picture” of the soil properties at a glance, but either method can be used.

We now have a description of our soil, phosphorus is low, so how much should we apply? This depends on the crop to be grown. For example, lettuce needs more phosphorus than banana grown on the same soil. The laboratory usually gives a GUIDE as to how much should be applied for the selected crop, but this should not be taken as a commandment, it has to be modified to suit your actual circumstances.

One major factor to consider is your yield goal. If you have good growing conditions, high yielding varieties, proper drainage, irrigation, pest and disease control, you can achieve high yields. This requires a good supply of nutrients and therefore fertilizer application rates should be high. However, if your field has problems that you cannot solve (like no irrigation in a low rainfall area) and you know that you will not get high yields, the fertilizer rate should be reduced as the crop will not make use of large amounts of nutrients. The laboratory does not know your field conditions and cannot make these decisions for you. In some cases, the lab will ask for your yield goal and adjust the recommendations to suit, in other cases, they may give a generalized recommendation that you must adjust yourself.

The economics of fertilizer use is another factor that the laboratory cannot decide for you. Some growers apply high rates of phosphorus when the fertilizer price is low. When the price is high, they reduce their rates and rely on the residual nutrients from the previous applications. This works with nutrients that do not move in the soil, but cannot be used for nitrogen application as the excess nutrient does not remain in the soil for the next crop. 

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