Soil Test Science
By: Robert N. Smith
The importance of testing your soil before planting food plots has been stressed in many previous articles in Quality Whitetails. The process is quick, inexpensive, and makes a world of difference in the quality and productivity of your plots.
Most serious food plot planters test their soil before planting. However, once they receive the soil analysis back from the lab, many are left scratching their heads trying to make sense of the report and calculate appropriate lime and fertilizer requirements. This article will detail how to read a soil sample and implement the recommendations in the field.
Before outlining how to interpret a soil analysis, some background information is needed. Plants can be thought of as nutrient transfer agents to get nutrients from the soil into the deer. The condition and quality of the soil affects how well plants can transfer nutrients. Managing soil fertility impacts how well plants grow, and how nutritious they are. In brief, collect six or more samples from each plot to be planted and mix these in a plastic or glass container. Then collect a single subsample from this mixture. The samples should be collected diagonally across the plot to ensure all soil variability is accounted for. Records should be kept for reference when collecting future samples from the same plot.
Sampling depth is one of the most important aspects of soil testing. Soil levels can vary greatly between three and six inches deep. A core sampler which collects 4-inch samples is the most commonly-used sampling tool, although a shovel or hand spade can do the job. After each food plot has been representatively sampled, the sample should be labeled with location and type(s) of seed or seed mix to be planted. The samples are usually sent to a Cooperative Extension Service, university, or private agronomic laboratory for analysis. Some fertilizer companies offer free analysis to their customers, while most other labs charge a nominal fee of $5 to $12 per sample.
It is important to remember that different plants have different nutrient needs. This is especially important when planting mixes containing both broadleaf plants like clover, and grasses like wheat. With a little experience or professional advice, you will soon learn which plants have the highest lime and fertilizer requirements.
Soil tests are performed on subsamples to estimate soil acidity (pH) and the amount of available nutrients in soil. The results are then summarized and printed. The difference between the available nutrients and the amount needed by the plants for optimum production is then recommended. Lime and fertilizer amendments are generally grouped for food plots with similar soil test results.
SOME SIMPLE SOIL FERTILITY CONCEPTS
Some soils can retain and transfer nutrients better than others. Lime and fertilizer applications are the most efficient in the best soils, so selecting high quality sites for food plots is important. Two important soil fertility characteristics are 1) how much total nutrition the soil can hold and, 2) how much of that nutrition is actually available. Soil nutrients may be lost or made unavailable through leaching, plant uptake, browsing (physical removal), volatilization, denitrification (loss of nitrogen), acidification, microbial uptake, and erosion.
Soil scientists use the term "cation exchange capacity" to explain how much transferable nutrition soils can sustain. Organic matter and soil texture are two important determinants of cation exchange capacity.
Decomposed organic material in the soil increases the soil¹s ability to hold water and nutrients. Though it may be beneficial in the long run, putting large amounts of fresh organic matter (e.g., manure) on the soil can decrease the available nutrients, especially nitrogen, in the short term as microbe populations break down the complex carbon molecules. This short-term utilization of available nutrients by decomposer organisms is why organic gardeners compost material before adding it to the soil.
Soil texture (size of particles) is important because it determines the surface area available to hold soil water through surface tension. Size also controls the sites that available nutrient ions can be held onto the soil. Clays are the finest textured particles, sands are the largest, and silts are intermediate. To help envision the impact of particle size, think about sand as double 00 buckshot (9 pellets in a standard 12 gauge shell) and clay as number 8 shot with 238 pellets in a comparable load. A cubic foot of sandy soil can have a surface area of 0.9 acre, while soils with more silt and clay can have three acres or more of surface area in a cubic foot. In brief, the greater the surface area of the soil, the more nutrients and water it can hold. While a soil such as clay may contain a large amount of nutrients, if the nutrients are not available for plant uptake, neither plants nor wildlife benefit. For nutrients to be available, there needs to be 1) adequate soil moisture, 2) good soil tilth (fluffy but firm), 3) a soil microbe population in balance with organic matter, 4 )suitable pore space and aeration, and most importantly, 5) a proper pH for the crop being grown.
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.
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 don¹t reliably produce forage for several years.
The type of lime or fertilizer used usually depends on three factors: 1) the material¹s 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.