About Plant Nutrients
Only 16 elements are known to be required for plant growth. These nutrient elements can be divided into four groups:
- Macronutrients - Carbon Dioxide and Water
- Carbon (C)
- Hydrogen (H)
- Oxygen (O)
- Primary Nutrients From the Soil
- Nitrogen (N)
- Phosphorus (P)
- Potassium (K)
- Secondary Nutrients From the Soil
- Calcium (Ca)
- Magnesium (Mg)
- Sulfur (S)
- Micronutrients From the Soil
- Boron (B)
- Zinc (Zn)
- Manganese (Mn)
- Copper (Cu)
- Molybdenum (Mo)
- Iron (Fe)
- Chloride (Cl)
The nutrient elements C, H, and O are obtained by plants from air (CO2) and water (H2O) in sufficient amounts to support maximum growth. There is no need for concern about supplying these elements in fertilizers.
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The elements N, P, and K constitute about 3 to 5% of the dry weight of most plants and are the most frequently deficient nutrients in Alabama soils. Accordingly, these are the nutrients in fertilizers most frequently and abundantly applied.
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- Nitrogen is the nutrient that is most frequently limiting crop production and is needed in greates quantities for most nonlegume crops. Soil tests are not reliable for determining the nitrogen supplying capacity of individual soils in the Southeast. There are several reasons for this. First, nitrogen is stored in the soil's organic matter. The rate of N release for crop use is affected by organic matter content, temperature, moisture, length of growing season, and other factors which make it impractical to predict the amount that will be supplied by the soil for a growing crop. Second, Alabama soils are low in organic matter and do not vary much in their capacity to supply nitrogen. Therefore, nitrogen recommendations are based primarily on the crop to be grown.
The most economical rates of application have been determined in numerous field experiments. The amounts recommended should be adjusted by growers based on experience with rates used previously. Nitrogen supply is the dominant fertility factor in determining rate and amount of growth of most crops. Legume crops get most of the N required for their growth from the air, which contains about 80 percent N.
- Phosphorus content of practically all Alabama soils originally was low. Fertilizers used in the early days of fertilization were much higher in P than in N or K. Fortunately, P does not leach through soils but forms compounds with other elements in the soil and is released slowly. However, P is lost from fields where the surface soil is allowed to erode. Also phosphorus in the plow layer may become diluted when turning the land deeper than normal, resulting in possible phosphorus deficiencies.
Crops require much smaller quantities of P than of N and K. Usually plants contain about .2 to .3 percent of P. Therefore, under continuous fertilization, soil content of P has increased on many soils to High levels. About 50 percent of all samples received by the Soil Testing Laboratory in recent years have been High in P, and crops grown on those soils would not be expected to respond to P applications. Experiments at several of the substations and experiment fields have shown that where P has been built up to High or Very High levels, they may go for 40 years or more without reducing yields of most crops from lack of P. Therefore, on soil test reports for field and forage crops, none is recommended High levels, but growers should to sample each year where none is applied to avoid any loss in yield should soil P drop back into the Medium range, where some response could be anticipated.
- Potassium. As yields have been increased by higher N and P fertilization, the need for K on some soils has increased. Most of the sandy soils of Alabama are deficient in K, while the clays and clay loams are less likely to be deficient. Response to K has been determined in numerous experiments throughout the State. Excessive use of K may cause Mg deficiency, especially on sandy soils. Recent residual studies have shown that K may accumulate in most soils where recommended rates are applied. When soil tests indicate that this accumulation has reached the High level, growers may reduce or cease applications until the soil level drops back into the Medium range.
The elements calcium (Ca), magnesium (Mg), and sulfur (S) are classified as secondary nutrient for plant nutrition. Some Alabama soils and crops should be supplied with these nutrients.
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- Calcium (Ca) is supplied in both calcitic and dolomitic lime. Where soil pH is maintained in the proper range, calcium deficiency is not likely to occur. Peanuts, tomatoes, and peppers are the only crops which have been found to suffer from lack of calcium in Alabama fields. This deficiency may result in unfilled pods (pops) in peanuts and blossom end rot in tomatoes and peppers. All samples are analyzed for calcium in the Soil Testing Laboratory. When the tests indicate that deficiencies may occur on these crops, gypsum may be recommended if lime is not needed.
- Magnesium (Mg) content of all soil samples is also determined. Some sandy soils in Alabama are deficient in Mg and these soils are usually low in pH. The most practical way to prevent Mg deficiency is by using dolomitic lime when soil tests indicate that Mg is Low. On soils where Mg is not found to be deficient, calcitic or dolomitic lime is satisfactory.
- Sulfur (S) deficiency has increased as high analysis fertilizers made from ammonium phosphates have replaced fertilizers made from superphosphate, which contains about 12% S. Sulfur added to soils in rain has decreased in areas where effluvium from industrial plants has been scrubbed to reduce air pollution. Sandy soils of the coastal plains and sandstone plateaus are most likely to be deficient in S, because this element may be leached from sandy surface soils. There is no practical soil test procedure to use in predicting S deficiency. It frequently accumulates in the subsoil and plants may recover from the deficiency when roots reach the subsoil. It is recommended that all crops receive an average of about 10 pounds of S per acre per year, applied in fertilizer or in pesticide applications. Crops most likely to respond to S fertilization on sandy soils are wheat, corn, cotton, and vegetable crops. Farmers should always check when buying mixed fertilizer to be certain that it contains enough S to supply this amount. When fertilizers are not applied each year, fertilizers should contain enough to last until the next application.
Although the seven micronutrients are as important in plant nutrition as the primary and secondary nutrient, they are needed in much smaller quantities, and most Alabama soils contain adequate amounts for most crops. The Department of Agronomy and Soils of the Agricultural Experiment Station has conducted field, greenhouse, and laboratory research continuously since about 1930 on the response of crops to micronutrient elements. Field experiments with boron, zinc, manganese, copper, iron, and molybdenum have been conducted with various crops on the substations, experiment fields, and on farmers' fields throughout the State. Whereas some crops may use between 20 and 200 pounds per acre of N, P, K, Ca, Mg and S, they use less than 1 pound per acre of the micronutrients. Most Alabama soils have an abundance of minerals containing micronutrients. In some cases, over application of micronutrients (e.g. Zn, Cu, B) could lead to a toxicity. Metals such as Zn and Cu build up in the soil. These may also be found in certain pesticides. Soil test for Zn and Cu may be more valuable for avoiding toxic buildup rather than for predicting deficiencies.
Research with field and forage crops has shown that most deficiencies of micronutrients are limited to boron and zinc for a few crops on certain soils. The most practical recommendation for these nutrients is to apply them to specific crops in all cases or on soils that may be deficient. This eliminates the need for soil tests that are, in most cases, expensive and not always dependable enough to serve as a basis for recommendations. Although some soil testing laboratories determine micronutrient levels in the soil and make specific recommendations, their interpretations are not based on research for Alabama soils. A chemical analysis or soil test is of little or no value unless calibrated against crop responses to applications of the nutrient in question. Such information can only be obtained from fertility experiments conducted under a wide range of soil and climatic conditions. The following is a brief description of the Auburn Soil Testing Laboratory's recommendations for the micronutrient elements.
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- Boron (B) is recommended for cotton, peanuts, clovers grown for seed, alfalfa, cauliflower, broccoli, root crops, apples, pears, and plums. Analysis for B is expensive and the cost of applying recommended rates is quite low; therefore, recommendations based on needs of specific crops are more practical than routine soil analyses.
- Zinc (Zn) is recommended for corn on sandy soils where the pH is above 6.0 or for the first year after applying lime. It is also recommended for peaches, pecans, apples, and pears. These are the only crops that have responded to Zn on Alabama soils. Zinc deficiency in corn seedlings is likely to occur in cool, wet seasons. Corn plants usually recover when warm weather arrives but yield may be decreased by the early deficiency. Routine analysis for Zn is not necessary in most cases, but soil and plant analysis for Zn may be helpful in diagnosing suspected cases of deficiency or toxicity. Simultaneous applications of excessive amounts of both lime and phosphorus can induce Zn deficiency on almost any crop. Soil tests showing pH values above 7.0 along with Very High or Extremely High P indicate a probability that Zn deficiency may occur on some soils.
Zinc toxicities could occur on sensitive crops such as peanuts where excessive Zn application have caused high soil Zn levels (> 10 mg/kg extractable Zn) on sandy soils. Maintaining a soil pH above 6.0 may help to reduce Zn toxicity symptoms.
Broiler litter, certain pesticide, and some industrial by-products used as soil amendments may contain high concentration of Zn.
- Iron (Fe) is a common deficiency for only a few crops (e.g. soybeans) on the high pH soils of the Black Belt and for some specialty plants (e.g. azaleas, centipedegrass, and blueberries) where lime or phosphorus is excessive. This deficiency cannot be corrected by application of Fe to the soil, but can be corrected on ornamental and fruit crops by spraying with a dilute iron solution. Soil analysis for Fe is worthless. Some soybean varieties are less susceptible than others to Fe deficiency. Varieties rated for susceptibility to Fe deficiency can be found in soybean variety reports.
- Molybdenum (Mo) application to soybeans as a foliar or seed treatment at planting is recommended for all soils of North Alabama and for Black Belt soils. Deficiency of Mo on soybeans on acid soils can usually be prevented by liming. Because Mo is needed in such small quantities Soil Testing is not recommended.
- Manganese (Mn) is high in almost all Alabama soils and is not recommended for any crop. Soybeans grown on sands with poor internal drainage, high organic content, and a pH above 6.0 may show Mn deficiency. Symptoms of cyst nematode damage are very similar to those for Mn deficiency on soybeans.
- Copper (Cu) and Chlorine (Cl) have not been found to be deficient for any crop on Alabama soils. There is no need to supply these elements in fertilizers in Alabama. Excessive Ca may be applied to soil in broiler litter and certain Cu-containing pesticides.
Present Policy on Micronutrients
Due to the points listed above, the laboratory does not analyze soil samples for S, Mn, Cu, Cl, Fe, or Mo. If and when research shows that analysis for one or more of these elements is justified, this service will be provided.
Under certain conditions, a soil test for B and Zn may be helpful in diagnosing problems. Therefore, samples will be analyzed on request for B and for Zn at an extra charge. Soil Test reports may have to be delayed for several days when these elements are being analyzed.
Last Updated: Feb. 24, 2011