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Soil Test Science - Part 2
This article is a continuation from Soil Test Science - Part 1
LIME
Soil acidity impacts nutrient availability. In an acidic soil (low pH),nutrients are bound to soil particles making them unavailable to the plants. If fertilizer is added to an unlimed acidic soil, only a portion becomes available. The remainder, sometimes the majority of the nitrogen and phosphorus, is bound to the acidic soil particles. Lime is the agent that can change the nutrient balance and free nutrients for plant uptake. As pH increases up to the optimum range (generally between pH 6-7) for the crop, nutrients become more available. In addition, since most soil fungi and bacteria cannot tolerate acidic conditions, they do not break down organic matter efficiently in an acidic soil. The addition of lime allows these microbial populations to flourish and release the tightly bound nutrients. The result is an indirect fertilization. Two important characteristics of lime are the kind of lime and the size of the lime particles. All agricultural liming materials are compared to calcium carbonate to allow for comparison. If magnesium is needed in the soil, then dolomite is the preferred liming material. With all other factors being equal, the finer the lime is ground, the more rapidly it acts and the more thoroughly it is mixed into the soil. Conversely, the more grinding done, the higher the cost and the more rapidly its effects are lost. Lime particles larger than half an inch are practically useless, while those smaller than 0.06 of an inch are 100 percent effective. Most agricultural limestone generally passes through a half-inch mesh screen with 25 to 50 percent passing through a 0.06-inch mesh screen.
Similar to many nutrients, lime can be leached from the soil. This happens most rapidly in warm regions with abundant rainfall and especially on sandy soil. Many premium forage plants, such as alfalfa and some clovers, require a pH of 6.0 or higher. If you are trying to grow one of these plants, choose soils with a clay component and lime regularly. While some food plots may only need liming every two or three years, on some sites it is necessary to lime annually, especially for the first few years after a new food plot is created. Lime is also needed more frequently in areas where high amounts of the ammonium form of nitrogen are added, where crops with high calcium and magnesium needs are planted, and where maintenance of a high pH is critical for crop production.
SOIL NUTRIENTS
Similar to the test for soil pH, tests for nutrients involve extractingthe available nutrients from a known quantity of soil in a water mixture. The nutrients most commonly tested for include phosphorus, potassium, calcium, and magnesium. Micronutrients that may be tested for include boron, zinc, copper, manganese, and iron. Tests for nitrogen and sulfur are generally not conducted because the rate at which they are released from soil organic matter or transferred into gaseous form cannot be reliably predicted. In areas where soils are highly deficient in sulfur or calcium, highly concentrated fertilizers are not recommended because they rarely contain these elements.
SOIL TEST INTERPRETATION AND USE
Soil test results are usually returned with one page for each sample submitted. This page will contain a summary of the available nutrients in the soil and may include a table and/or bar graphs showing whether the sample has a low, adequate, or high availability for each nutrient tested. The results will also contain a recommended lime and fertilizer amendment to enhance the soil for the plant(s) of interest.
The amounts of nitrogen and sulfur recommended for a specific crop are based on fertilizer trials in the region of the state where the soil sample originated. Since nitrogen can easily be lost, recommendations often call for a split application - half at planting and half just before major plant growth. Micronutrients are generally not added unless the soil test indicates there is a deficiency. Specific tests for micronutrients are typically done when plant deficiency symptoms (typically yellow or purple coloration or irregular growth habits) indicate a problem. Soil test results from all food plots should be divided into groups needing similar amounts of amendments. Unless you only have a small number of food plots, it usually is not cost effective to treat each food plot with exactly what it needs, so a general prescription is made for each group of similar food plots. These soil test results will usually be grouped by some correlated characteristic, like hills and bottoms or new food plots and old food plots. With new food plots, it can take five to eight years of high annual nutrient amendments to build up the available nutrients before getting onto a maintenance application of lime and fertilizer. Considering the topographic position, soil color, and soil texture can help you logically group your plots.
When developing a recommended amendment for a group, it is important to keep the nutrients in balance. Too much of one nutrient may interfere with availability or uptake of other nutrients or leach into adjacent water. Adding too much of some nutrients can result in plant death or lack of germination. For example, over-application of animal waste high in manganese can result in manganese toxicity and food plots that donšt reliably produce forage for several years.
The type of lime or fertilizer used usually depends on three factors: 1) the materialšs ability to provide the needed nutrient changes in the soil, 2) material cost, and 3) transportation and spreading difficulties and costs. For small or isolated food plots, getting the material to the plot and spread is usually critical. It is difficult or impossible to get commercial applicators to take large equipment, sometimes even tractors, into some small food plots. If transportation or spreading with small equipment or by hand is involved, using concentrated materials that may cost a little more per ton may be justified.
For practical purposes, the minimum amount of lime to spread is two tons per acre since this will not "overlime" any food plot that shows a need forlime through a soil test and it optimizes the use of transport and spreading equipment. From an economical standpoint, the best time to lime is just before or after most farmers in your area have applied lime. Lime contractors would much rather work on large agricultural fields than in small scattered food plots, so it can help to offer them work while their equipment is idle. For most nutrient efficiency, especially alfalfa or other pH dependent plants, lime should be applied five to six months prior to planting to allow it to have time to impact soil chemistry.
If the soil test calls for two tons of lime equivalent, then you need two tons of calcitic limestone, 1.8 tons of dolomitic limestone, or 3.3 tons of basic slag. This is calculated by taking the tons needed and dividing by the percent CaCO3 equivalency in decimal form (1.10 for dolomitic lime). For example two tons divided by 1.1 CaCO3 equivalents equals 1.8 tons. If the recommendation is to apply 80 pounds of nitrogen, 80 pounds of phosphorus, and 80 pounds of potassium per acre, the nutrients could be supplied in 800 pounds of 10-10-10, 615 pounds of 13-13-13, or 400 pounds of 20-20-20. Obviously, if you are spreading this by hand, then you may want to consider using 20-20-20, even if it is more expensive.
Calculating fertilizer rates is done by dividing the recommended application rate by the fertilizer analysis (number on the bag for that nutrient) multiplied by 100. For instance, if using the fertilizer 0-0-60 (0 percent N, 0 percent P2O5, and 60 percent K2O), and the recommended application rate is 120 pounds potassium (K2O) per acre, then the rate is:120 pounds K2O needed divided by 60 pounds K20 times 100 which equals 200 pounds of 0-0-60 per acre. This calculation would be done for each nutrient to be added, with slight over-application preferred.
When dividing food plots into groups for treatment and deciding what materials to use in the food plot, it often pays to work through this process with your local fertilizer applicator. It is in their best interest to help you manage your soil fertility since lime and fertilizer application is a recurring need. Cooperative extension service agents or other knowledgeable professionals also can provide recommendations.
Soil tests are not infallible, but the recommendations are generally pretty good and far better than any "seat-of-the-pants guess." Keeping copies of the soil test recommendations and watching the success of the food plots after following the recommendations can enhance your confidence in this important, but often overlooked, tool.
By: Robert N. Smith
Hits: 1182
Posted : 02/12/2006
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