Author: Mr. Guy Sela, CEO of SMART! Fertilizer Management software and an international expert in plant nutrition and irrigation.
Soil testing is an important tool for evaluating or avoiding problems of nutrients balance.
Soils are the source of thirteen of the sixteen essential plant nutrients. Plants absorb available nutrients, which can be replenished by adding fertilizers.
To achieve good yield and quality, nutrient balance has to be maintained. Nutrient imbalance may result in deficiencies, toxicities or interference of one nutrient with the absorption of others. This may result in stress to the crop, causing a decrease in quality and/or yield.
However, reading and interpreting the soil test results might be very confusing and challenging.
Example of a soil test report. Click image to enlarge it.
Growers can roughly estimate how much fertilizers should be applied to their crop according to general fertilizer recommendations. But a more accurate, cost-effective fertilizer application requires soil testing.
Soil test allows you to know the starting point, and this is a very valuable piece of information.
Here are the basics of how to interpret soil test results.
Taking a soil sample correctly is the number one step for getting reliable soil test results.
Soil sampling should be well planned and preformed. For example, the sample site should be far from roads, fences, tree groups, piles of fertilizers and manure or any other object that can locally affect the soil properties and content.
The soil sample should represent the entire field as closely as possible. If the field is not uniform, and consists of different areas with different properties, each area should be sampled. Compiling results from completely different areas and averaging them into one report, will obviously give us a very misleading result.
There are many factors that affect the plant growth. A Soil test does not give answers to poor or inadequate conditions, such as critically low or high temperatures, inadequate drainage, plant diseases, pests damage, competition with weeds etc.
Once you took the soil sample correctly and you acknowledge the many factors that may affect your crop, you can proceed to interpret the numbers in the soil test report.
Soils usually contain much higher amounts of nutrients than what you see in the soil test report, because lab results describe only the available nutrients to plant. To estimate the amounts of nutrients which are actually available to plants, different testing methods were developed. Many of these methods give empirical values or measures.
Soil test reports might be confusing: different labs use different testing methods, resulting in different results for the same sample!
For example, phosphorus is often measured using "Olsen" testing procedure or "Bray" testing procedure.
The testing method used by the lab is often mentioned in the lab test report. It is essential to know which method was used by the lab for each element tested, in order to interpret the results.
Labs may even use the same methods, but express the results in different units, or give a different interpretation to the same result.
To correlate the numbers on the soil test report with crop response to added nutrients, numerous field experiments are required. The reason is that the same soil test results might lead to different recommendations in different soils, different areas and under different field conditions.
The numbers on the soil test report, together with the description of the nutrient content in the soil, can indicate if the predicted crop response to fertilizers will be favorable or not.
Good labs, having good experience, use their regional database to give a description of the level of each nutrient (indicating if its level is too high, too low or adequate) and even fertilizer recommendations.
You can read here about the different approaches for giving fertilizer recommendations.
The soil test report, together with the grower's close familiarity with his crop and field conditions, give the grower the starting-point for deciding on a fertilizer program.
Soil testing gives makes a good starting point for making better fertilizer management decisions. The soil test results should be put in context and their interpretation should be adjusted to the individual crop behavior and specific field conditions.