Fertility Management
Soil Testing
Soil testing is available from state soil testing laboratories and many private testing services. Regular testing not only indicates current soil fertility problems, but also tracks fertility changes over time. In some cases, soil testing saves money by reducing fertilizer use. In the south, most soils that have been in production for long periods are quite high in phosphorus (P) due to years of fertilizer application and low phosphorus mobility in the soil. In 1992, 60 percent of sweetpotato fields, 50 percent of cucumber fields and 70 percent of bell pepper fields in North Carolina needed no additional phosphorus because of the large quantity in the soil.However, undisturbed or non-agricultural soils in the southern United States are frequently low in phosphorus.
Limitations. Soil test recommendations are usually not cultivar specific. Cultivars vary in their ability to scavenge soil nutrients and some may yield well at less than recommended levels.
Soil testing does not provide information about soil structure, nitrogen status, or biological activity, although some estimate of soil organic matter is usually included. The soil's ability to hold nutrients is reflected by soil test measures of cation exchange capacity (CEC). When fields vary naturally in texture and nutrient content it is difficult to get a representative sample for testing. To overcome this potential problem, guidelines have been developed regarding the depth and number of subsamples collected over a given area. At least 15 to 20 soil cores per sample are usually suggested to obtain a representative sample.
Past field history should be considered when interpreting soil test results. This is particularly important when past fertilizer additions have been in the form of organic materials which release nutrients slowly. When such materials are present, soil and laboratory tests may underpredict the amount of soil nutrients actually available to plants over the course of the entire season.
Tissue Testing
Most soil testing services also offer tissue testing. Regular tissue testing is a vital management tool for growers in certain highly controlled environments such as hydroponic vegetable production in greenhouses.
In these environments, plants are totally dependent on a constant supply of nutrients and are likely to suffer if they experience even a temporary deficit or imbalance. Outdoors, growers who apply fertilizer through drip irrigation lines also frequently depend on tissue testing to make fertilizer decisions. The need for tissue testing is less obvious for growers who use slow-release composts, cover crops and manures since fertilizer additions are usually made annually rather than daily or weekly.
On the other hand, tissue analysis will help identify toxicity from micronutrients such as copper in poultry litter. Tissue testing will also guide growers in deciding whether to make additional applications of a high-nitrogen manure. In crops such as lettuce and spinach, excess leaf nitrate can cause marketing problems. Baby food manufacturers, for example, have strict guidelines for nitrate levels present in spinach to be processed.
All growers will find tissue sampling valuable if they suspect nutritional problems. For this type of problem, it is important to sample both soil and tissue, since the problem may be nutrient uptake rather than nutrient availability. For example, high salts levels in the soil may damage roots, reducing nutrient uptake directly or by subsequent pathogen infection.
Petiole sap testing
Growers who suspect that they have nutritional problems often need an immediate diagnosis. Petiole sap testing kits can be used in such cases for on-farm analysis of nitrate-N and potassium levels. This information would be helpful, for example, to a grower making weekly adjustments in fertilizer injected into drip irrigation systems.
When equipment is properly calibrated, results from petiole sap tests are well correlated with laboratory results. Petiole sap recommendations based on work in Florida in broccoli, eggplant, and potatoes are given in the chapters describing cultural practices for those crops. The ion-specific electrodes used in testing N and K in petiole sap usually cost about $300. Test chemicals for calibration must also be purchased.
Sources of Minerals
Nitrogen. Most organic materials contain nitrogen (N), but the percentage composition varies widely. Average Nitrogen (N), Phosphate (P2O5), and Potassium (as K2O) percent composition of various materials . Some of the materials listed, such as apple fruit, contain only trace amounts of N, while others, such as bloodmeal, are relatively high in N. Nutrient sources lists the N percentage and the relative risk of leaching and volatilization (loss in gaseous form) of selected fertilizers.
In interpreting data from the Nutrient sources table, note that the leaching risk for urea and ammonium fertilizers is low only as long as the N remains in the ammonium form. In soils with high biological activity, nitrifying soil microorganisms can rapidly convert ammonium to the highly leachable nitrate form. Nitrogen availability may be reduced, however, by lower biological activity in cold or extremely dry or flooded soils.
Phosphorus. Phosphorus is also available from organic matter additions; P availability increases when conditions are favorable for microbial activity and root growth. In cool weather, lower biological activity results in less P being available than in the same soil under conditions favoring biological activity. Cool soil temperatures also restrict root growth which reduces the amount of nutrients taken up by roots.
In the soil, P is found as reaction products which have precipitated out of solution and as held or adsorbed forms on surfaces of various soil minerals. Soil testing before adding P is particularly important because phosphorus levels of southern coastal plain soils that have been farmed for many years may not need additional phosphorus for several years. On the other hand, southern soils which have not been fertilized are often quite low in P.
Since phosphorus has only limited mobility in the soil, incorporation is a better application method than side dressing or broadcasting. surface applications. Sidedressing P may concentrate rooting in the area fertilized, thereby increasing the need for irrigation because of restricted root growth outside the area of P application.
Potassium. In the soil, potassium is held by adsorption on soil particles, cation (charged particle) exchange, and inclusion in other reserve pools. When root development is good, plants are better able to exploit potassium reserves in the soil. Organic material is generally a good source of potassium .
Although granite dust and greensand contain only a small percent of immediately available potassium and some trace elements, they release potassium slowly through mineralization. These materials are most beneficial when used to build reserves in the soil.
Lime and other calcium sources. Agricultural lime, chemically described as the oxide, hydroxide, or carbonate of calcium and/or magnesium, is used to correct soil pH. The best forms of lime to use depend on the situation. Both dolomitic and calcitic limestone have approximately the same liming capability, but dolomitic limestone has a higher magnesium content and is useful where soil magnesium levels are low. The amount of lime to apply should be determined by a soil test. Although agricultural lime is the most commonly used method of raising soil pH, finely ground shells, wood ash, and other materials will also raise soil pH levels.
Gypsum (calcium sulphate) supplies calcium, but has no effect on soil pH. Gypsum can reduce erosion on heavy clay soils by improving soil structure and increasing water infiltration. To reduce erosion and soil crusting, gypsum is spread over the soil rather than incorporated. Fine particle size gypsum is most effective.
Because gypsum is more soluble than limestone, the calcium in gypsum leaches through the soil and increases calcium availability at deeper levels in the soil. Adding calcium in the subsoil can help reduce aluminum toxicity. This will allow root growth at deeper levels, which improves drought tolerance. There may be little or no effect the first season, but beneficial effects of gypsum on yield in calcium-deficient soils may persist for six years or more after application. However, calcium interactions with other cations should also be considered. Calcium from gypsum may replace magnesium and potassium in the topsoil, which makes deep, sandy soils, which have a low Cation exchange capacity, vulnerable to loss of magnesium and potassium by leaching at high calcium levels. Because of these interactions, timing of lime application in relation to other fertilizer applications can be important.
A Florida study showed that applying lime four weeks before phosphorus application (in the form of diammonium phosphate or triple super phosphate) increased dry weight of snap beans compared to applying lime four weeks after phosphorus application. P resumably in the late application, calcium phosphate formed and precipitated out of the soil solution where it was less available.
Secondary nutrients and micronutrients. Secondary nutrients (calcium, sulfur, and magnesium) and micro-nutrients (boron, copper, iron, manganese, molybdenum and zinc and chlorine) are essential for growth, but required in smaller quantities than N, P, and K. Additions of micronutrients should be made only when a deficiency is indicated by a soil test or plant tissue analysis. See Degree of sensitivity of various plants to deficiencies of micronutrients for information on sensitivity of vegetable plants to micronutrient deficiency. Because only small amounts of micronutrients are required, uniform and exact application is important. Significantly exceeding the application rates indicated by soil or tissue tests is likely to injure the plants.
Many factors determine whether organic matter increases or decreases the availability of micronutrients. Metallic cations, including iron, manganese, zinc and copper, can be bound to chelates (organic compounds that are able to bind tightly with cations). In this form they are partially protected from changes in pH and from forming precipitates. Iron, for example, is typically added to fertilizer solutions in chelated form. However not all organic compounds that chelate micronutrients make them more available. Micronutrients may be bound so tightly to organic matter in organic soils (greater than 18 percent organic matter), that crops suffer micronutrient deficiencies. Copper is one micronutrient that is often unavailable and deficient in organic soils. With some nutrients, however, lack of availability is an advantage because it reduces potentially toxic levels of micronutrients. Manure additions, for example, rarely lead to toxic levels of micronutrients even though fairly high levels of micronutrients are present in manure.
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