Wise farmers and ranchers care for the soil because they know that man is dependent on the top 6 inches (15.2 centimeters) of soil. In the plant-animal-soil continuum, soil is often neglected because it does not indicate stress in an obvious way. Animals and plants show physical symptoms but the soil must be looked at more carefully to monitor good health.
Importance of Soil Fertility
Soil that is rich in nutrients is fertile. The expectation of growing plants as food for livestock must include the reality that plants will take nutrients out of the soil. Replacing nutrients is the basic goal of fertilization.
Soils feed the plants which in turn feed the animals that feed us. Soil provides the support or foundation for plants and most of the nutrients. Soil is accumulated decomposing plant and animal matter with aging parent material. As the soil components break down, elements are released and become available to plants as nutrients. However, this process takes a long time and the soil will only be a result of the parent material, climate, those living organisms once living there, topography, and time. So what is made available to a plant at a certain time may not be exactly what a growing plant needs. Fertilization is supplementing the existing soil with additional, needed nutrients. Fertilizing wisely increases yield, quality, and profit.
Major Elements for Good Soil Fertility and Plant Growth
Plant growth requires a compatible relationship between the plant, the atmosphere, and the soil. The soil provides support and nutrients for plant growth. The air provides CO2 for photosynthesis and N2 for nitrogen fixing plants. Over 50 different factors enter into the relationship of plants, atmosphere, and soil. Some cannot be easily modified, like relative humidity, but many, like soil texture, can be adjusted by the land manager. Profitable production is the result of careful management. One of the key factors that can be manipulated is nutrition supplied by elements.
Sixteen elements are considered essential for plant growth because they are involved in metabolic functions required in the life cycle of the plant. Some, like carbon (C), oxygen (O), and hydrogen (H), come from the air. Nitrogen (N) is made available to the plant from the air and soil. But most of the needed elements that are nutrients for plants come from the soil. They are not all equally important but all play a role in plant growth. Most needed are nitrogen (N), phosphorus (P), potassium (K), and sulfur (S). Others are calcium (Ca), iron (Fe), magnesium (Mg), manganese (Mn), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo), chlorine (Cl), sodium (Na), cobalt (Co), nickel (Ni), and silicon (Si).
Nitrogen is the most critical element for grass plants because it is often deficient and yields obvious benefits. N fertilization on forages generally increases yield and crude protein content of cool and warm-season grasses. Stored carbohydrates are reduced, which produces a more succulent plant. Plants normally contain between 1 and 5% N, absorbed as nitrate (NO3-), ammonium (NH4+) ions and urea. Nitrate is most often available but must be reduced to NH4+ or NH3 for plant utilization. Too much nitrogen may result in animal disorders related to high nitrate, alkaloid content, or hypomagnesemia. Some plants also are more susceptible to lodging, disease, or insect invasion.
Legumes can fix their own nitrogen from the atmosphere. Nitrogen application is not typically recommended for legumes. Adding nitrogen decreases the nodulation on legume roots and the amount of N fixed by the plants. There is no yield increase of tissue nitrogen percentage when legumes are properly inoculated. Legumes usually require more K, S, Mo, and B than grasses. Since the nutrient needs of grasses and legumes differ, use fertilizers to manage a grass/legume mixture. Adding K, S, Mo, and B will favor legume growth. Grasses crowding out legumes or the invasion of weedy species may indicate decreasing levels of K. K has shown to increase stand longevity in addition to yield and quality. When managing a mixed sward, fertilization can do more than just increase yield. Remember the differences between grasses and legumes. Determine which species are desired as dominant in a mixture. What kind of mixture best meets the animal requirements? Do other limitations determined by geophysical factors (soil, weather, elevation) favor grasses or legumes?
Phosphorus (P) makes up about 0.1 and 0.4% of a plant and is involved in energy storage and transfer, root growth, early maturation, quality, and disease resistance. Plants absorb H2PO4- or HPO42- orthophosphate ions. Adequate P is especially important to germinating seedling root growth.
Potassium (K) concentration in vegetative tissue usually ranges from 1 to 4% of dry matter. Plants absorb N and P in compounds but the K+ ion is absorbed as K+. K influences enzyme activity, water and energy relations, transpiration and translocation, and N uptake and protein synthesis.
Sulfur (S) is absorbed by plant roots as a sulfate ion SO42-. Elemental S can also be used as a fertilizer. Normally, S concentrations range between 0.1 and 0.4%. Deficiencies in S result in retarded growth: stunted, thin-stemmed, and spindly plants. Some confuse a S deficiency with a shortage of N because of growth problems and pale green color. Sulfur problems show up first in younger leaves while N deficiencies show up first in older leaves. S is essential for plant amino acid synthesis.
Testing for Nutrients
There are a few ways to determine what nutrients are needed: soil testing, plant tissue analysis, yield response, and crop removal. Yield response and crop removal take careful observation and a long time period. Plant tissue analysis is a good way to determine how much of a nutrient is being absorbed by the plants and therefore depleted from the soil. Determining what has been removed by analyzing the crop yield is a way of determining what should be replaced. Plant tissue analysis can provide valuable information but replacement of nutrients should also consider leaching and other losses. Calculating the pounds of nutrients removed from dry matter yield involves a simple equation. Multiply the average tissue concentration (determined by tissue analysis) for the specific element (usually N, P, K, and S) by the dry matter yield in pounds per acre. If the average tissue concentration for K is 1.85% and the dry matter yield of a hay crop is 8 tons, put the percentage in decimal form and the 8 tons into pounds, each ton is 2000 pounds (907 kilograms): .0185 X 16000 pounds (7257.5 kilograms) = 296 pounds (134.3 kilograms) of K removed.
A soil test is the best way to know if the soil can provide these elements to plants. A soil test is a chemical method of estimating the capacity of the soil to supply nutrients. Taken every 2-3 years to monitor soil acidity and fertility, a soil test can be a very helpful tool before forages are planted. Soil tests determine what nutrients are in the soil; not the plant uptake, which can be measured by plant analysis. Technology is developing more accurate ways of soil sampling resulting in efficient, environmental-minded, and scientifically sound fertilizer application. But the basic guidelines include several highlights. Soil test samples should be taken every 2.5 - 5 acres (1-2 hectares) and in areas of different geophysical features (hillsides, soil types, areas managed differently) to represent the entire field, but should avoid small unusual spots. Dig out a sample of soil from plow depth unless a shallower height is needed as in cases of renovation or established pastures. Avoid contaminating sampling tools with fertilizer or soils from other spots. Do not use galvanized, brass, or bronze tools when planning to request information on micronutrients such as zinc. Place the collection of samples into a clean container and mix thoroughly. Fill a sample bag with soil and fill out needed information. Do not use paper bags for the composite sample. Each sample should be about 1 pint of soil consisting of subsamples taken from 15-20 locations. Record information about the samples and locations. Keep a map of soil sampling over the years to develop a real sense of the soil history and potential fertilizer application. Label them before sending them to a soil testing service. Request what tests you want performed since each test costs money.
Soil Testing Results: If results show a pH too low for optimal production of the intended forage, lime may be a wise investment.
Plant growth can be greatly hampered by soil being too acidic. This is especially important when N fertilizers form acids. As N fertilizers are used to grow more food, soils are continually becoming more acidic. Soils with too much acid can cause several problems. Many forage plants, like alfalfa, have specific soil acidity tolerances (measured as pH) for productive growth, but as acids increase soil pH drops and growth is hampered. Nitrogen fixation is also hampered in very acidic soils. Acid soils have poor tilth and are poorly aggregated. Other nutrients are not as available to plants. The concentration of soluble metals in the soil may become toxic. There may be a calcium deficiency. Lime is used to reduce acids and make the soil more alkaline.
Lime application rate is determined using the lime requirement test (SMP buffer method). The SMP buffer test is named after Shoemaker, McLean, and Pratt, the soil scientists who published the method in 1961. For established perennial or no-till crops, a top-dress lime application (1 to 2 t/a) may be beneficial. When very different soils are present within a field, variable-rate lime application is usually advantageous.
Limestone reacts only with the nearby soil, so mixing the limestone into the soil rather than leaving the limestone on the surface is necessary. Disking alone is not enough and may result in green stripes in plant growth. Disking followed by plowing, and further working in of the limestone is best.
Lime is beneficial when mixed well into the soil where seeds are germinating and plant roots growing. Lime should be placed deep enough to serve these purposes.
Lime applications can be expensive. Some alternatives may provide an inexpensive solution. Since lime encourages nodulation, using more inoculum may be helpful. Apply Mo at 4-5 oz/A, and P. Use species tolerant of acidic soils such as white clover or subterranean clover.
Fertilizer Management for Mixed Stands
Permanent pastures often mix a grass and legume to improve nutrition, utilization, and yield. Grasses are generally easier to grow than legumes because legumes have more narrow parameters like adaptation range and pH. Obviously, fertilizing for a single species would be much more simple. But legumes are generally higher in protein and mineral content, increasing the feed value. When a pasture is made up of grass, nitrogen is often the cause of limited growth. Having a legume to fix nitrogen and make it available in the soil for the grass is advantageous. If a pasture is more than 50% legumes, then N fertilization does not provide significant gains. Pastures containing more than 50% grass would benefit from N applications. But fertilization is a complex issue, as has been discussed, because the different nutrients have various relationships with each other. In order to make the best use of nitrogen the P, K, pH, must be kept in balance. The general rule is to fertilize for the legume.
Grasses are more tolerant of soil acidity than alfalfa and some other legumes. Liming should be considered for the legume requirements.
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