Understand Your Soil Sample Results
In January a Soil Fertility Workshop was held at Humboldt, with Dr. Charles Wortmann, Extension Soil Fertility Specialist, Gary Lesoing and Sarah Heidzig-Kraeger, Extension Educators in Nemaha and Richardson Counties respectively, presenting the program. One of the main areas of focus was understanding what soil sample test results mean, and recommendations based on the results.
Dr. Wortmann used an example of a soil test from one field that was sent to different soil laboratories and came back with different soil recommendations. The labs results were similar for soil analyses of the various nutrients and soil properties tested, but the recommendations were different. This is due to differences in philosophy. The University of Nebraska-Lincoln makes recommendations based on calibration of hundreds of soil tests taken over a number of years. Their recommendations are also based on deficiencies in the soil. These nutrient recommendations are usually sufficient to meet yield goals, but under high yield environments and certain field conditions, nutrient deficiencies could limit crop yields. Some of the other laboratories make recommendation on a building and maintenance program and others on a crop removal program. With these types of programs, nutrients such as phosphorus (P) and potassium (K) are built up to a certain level and then applied according to how much is removed from the previous crop. You should usually have sufficient nutrients in the soil, but you may never recover the cost of the fertilizer you are applying each year. In studies conducted at the University of Nebraska-Lincoln over several years at 3 locations, annual fertilizer costs per acre based on the University of Nebraska recommendations were significantly lower compared to other laboratories without affecting crop yields.
Another issue that was discussed was the impact of lime on crop yields. If soil pHs are low, soybean yields can be reduced and the availability of nutrients, such as P can be affected. An on-farm study conducted in eastern Nebraska from 1995 – 2005 showed the benefits and profitability of the addition of lime to a low pH soil in a corn-soybean rotation. By 1999 costs of lime application were recovered through increased crop yields, with a cumulative net return of $137 per acre by 2005. With no-till, it is recommended to apply ½ of the recommended lime amount on the surface, soil sample 4-5 years later and determine if more lime is needed at this time. If a field needs 4 tons/acre, apply 2 tons/acre, then check after 4-5 years and apply the other 2 tons/acre if the pH is low. The other concern is if a tenant is cash renting land and pays for liming. It is important that the tenant have an agreement with the landlord if the rent is terminated prior to getting full benefit of the lime, which may be up to 6 years or longer under no-till conditions.
Soil sampling can pay back a significant return on the investment in time and/or money tied up in it. By knowing the organic matter of the soil and also taking subsoil tests for nitrates, you may find you can have a nitrogen credit from the organic matter in the soil. Research from several years at the University of Nebraska has indicated that for each percent (1%) organic matter in the soil, 31 pounds of nitrogen from the soil will be mineralized (become available) for the next crop to use. If you deep soil sample down to 3 feet, you may also find residual nitrate in the soil profile, which may reduce nitrogen fertilizer requirements. It is doubtful that there will be much residual nitrate in the soil profile this year after a productive crop year and significant rainfall last summer, but under some circumstances, such as in a drought year, residual nitrate levels are significant.
Nutrient deficiencies in soil can also have a significant impact on yields of both corn and soybeans. While fall is a good time to soil test, you can still benefit from soil samples taken this spring. With the good crop yields the past years, several pounds of nutrients are being removed in the grain taken off at harvest. If your corn yielded 150 bushels per acre, on average you would remove about 20-30 pounds of phosphorus per acre. Soybeans that yielded 60 bushels per acre would remove about 20-25 pounds of phosphorus per acre. With these quantities of nutrients being removed from the soil, it is important to determine levels of nutrients in the soil and if fertilizer requirements will increase for this years crop. With today’s farm commodity prices and fertilizer costs, it is important to be as efficient as possible in crop production. Fields should be sampled periodically, every 2-3 years.
If you have questions on soil testing feel free to contact me at (402) 274-4755.
