SOIL FERTILITY

SOIL FERTILITY

Fertilizer requirements for each field should be determined on the basis of laboratory soil analyses. A balanced fertility program, with emphasis on available levels of phosphorous, potassium, magnesium, and nitrogen, is essential to high yields. Late January or early February are optimum times for performing necessary analyses. Following fertilizer recommendations based on the laboratory analysis for each field and yield goal is important. Heavy rates of potassium- containing fertilizers applied to the pegging zone can interfere with calcium uptake for developing pegs and pods. Soils of the eastern peanut-growing region of New Mexico typically have adequate available calcium in the pegging zone for Valencia varieties.

Certain micronutrients, including zinc, iron, manganese, copper, boron, and molybdenum, also are essential to peanut production. Boron deficiency impairs normal seed development and causes hollow heart. Hollow heart is an irregularly shaped blackened cavity on the inner face of the peanut seed. The condition is classified as concealed damage. Evidence from field studies indicates the symptoms of boron deficiency are more likely to occur at high yield levels. Soils testing less than 1 lb/A (1.12 kg/ha) should be considered for boron application. Only apply boron when there is a deficiency because too much boron can be harmful to yield and quality.

Zinc-deficient soils also can reduce crop yields. Soils are deficient if the DPTA extractable zinc concentration is less than 0.4 ppm. Zinc deficiencies can occur in alkaline soils that are low in organic matter and high in available phosphorous. In addition to adding zinc fertilizers, adding large amounts of organic materials, such as barnyard manure, and incorporating plant residues are also helpful. The most common recommendation for inorganic zinc fertilizer application is 6 to 10 lb/A (6.7 to 11.2 kg/ha). Zinc remains available in the soil for several years, although it may be tied up with phosphorus. Soils should be tested before adding zinc.

Symptoms of zinc deficiency can occur concurrently with symptoms of iron deficiency. The chlorotic strips of zinc deficiency are usually wider than those of iron deficiency on the portion nearest the petiole and may not run the entire length of the leaflets. High temperatures can cause zinc deficiency to appear as leaflet bronzing. Zinc-deficient plants also may be stunted.

Soils in eastern New Mexico's peanut-producing region are generally alkaline. Alkaline soils also may be deficient in copper, manganese, and available iron. Nutrient deficiencies, with the exception of iron, can be corrected by applying the required elements before or at planting time. Iron deficiency in peanuts can be corrected with foliar applications of iron chelates or iron sulfates. Applying iron through irrigation systems is not as efficient as foliar applications of chelates or sulfates. Soil applications of iron may cause the iron to be chemically bound and unavailable.

Although peanuts are legumes and can provide some of their own nitrogen (table 1), soil-available nitrogen is required by young seedling plants. Healthy, vigorous seedlings can be ensured with 10 to 20 lbs/A of starter nitrogen. If soil tests indicate this much nitrogen carry-over is available from the previous crop, additional nitrogen may not be necessary.

Most producers in eastern New Mexico use up to 100 lbs/A nitrogen fertilizer to ensure adequate nitrogen throughout the growing season. Although nitrogen boosts yields, additional nitrogen may increase disease problems by causing more extensive vine production, which results in canopy closure earlier in the growing season.

Table 1. Approximate plant nutrient content of a peanut crop.

Fertilizer	In tops	In pods	Total
Nitrogen (N)	90	75	165
Phosphorus(P2O5)	23	15	38
Potassium(K20)	80	11	91
Calcium (Ca)	45	2	47
Magnesium (Mg)	16	2	18
Sulfur (S)	14	4	18
Zinc (Zn)	0.2	0.01	0.21
Iron (Fe)	0.5	0.01	0.51
Manganese (Mn)	0.4	0.01	0.41
Copper (Cu)	0.025	0.005	0.03
Boron (B)	0.05	0.03	0.08
Molybdenum (Mo)	0.01	0.01	0.02
Chlorine (Cl)	Variable	-	-

Source: Peanut production in Texas, Texas Agricultural Experiment Station, Bulletin RM3 (1975).