Proper Fertilization for No-Till Corn and Soybeans Dr. John Bradley Conservation Tillage Specialist Monsanto Company Proper fertilization is a key aspect of reducing production costs and maximizing profits in no-till corn and soybeans. Using too little lime and fertilizer in your fields can reduce yields, while excessive use of these materials can increase costs without increasing production. Profitability could be reduced in both situations, so it is crucial to do an accurate assessment of what nutrients your fields need and then follow recommended application rates and timing.

The first step to proper fertilization is soil testing. A soil test provides guidelines for liming acidic fields, using molybdenum seed treatments and applying only those nutrients that will help produce better yields. Although fall is an excellent time to collect soil samples, fields can also be sampled in the spring. Be sure to collect samples early enough to obtain results before lime and fertilizer need to be applied. Also, make sure the sample is a good representation of the field that is in need of a fertilizer treatment recommendation. Contact your local county extension agent or agricultural chemical/fertilizer retailer for testing information and sampling instructions.

One of the most important soil chemical properties for soybean fields is soil acidity or low pH. Soybeans are very sensitive to acidic levels and usually respond favorably when lime is applied according to soil test recommendations. There are a few key reasons why applying lime to acidic fields may increase soybean yields: increasing nutrient availability, preventing manganese toxicity and increasing nitrogen fixation.

The availability of essential plant nutrients is optimum at a soil pH near 6.0. If soil acids become excessively high, nutrients become fixed or tied-up and unavailable for plant use. For example, the efficiency of applied phosphate and potash may be reduced as much as 50 percent when soil pH levels drop from about 6.1 to 5.0. Applying lime to acidic fields increases nutrient availability.

Applying lime to acidic fields may also prevent manganese toxicity. When soil pH levels drop to near 5.0, increased levels of manganese develop which may become toxic. As a result, soybean plants become stunted in growth, appear dark green in color, have small crinkled leaves and reduced yield potential.

An increase in nitrogen fixation is another reason to lime acidic fields. The soybean is a legume. If it is properly inoculated with nitrogen-fixing bacteria, it will produce its own nitrogen. However, acidic soils prevent these bacteria from multiplying and functioning properly. Raising the soil pH by applying recommended amounts of agricultural limestone creates an environment favorable to good nitrogen fixation.

Additional ways in which liming acidic soybean fields can help to improve yields are by enhancing activity of soil microorganisms and providing optimum pH ranges for availability of secondary and micronutrients. In some cases, the effectiveness of certain residual herbicides is also improved by liming acidic soybean fields. However, when no-tilling Roundup Ready[R] soybeans, residual herbicides are not recommended.

In soybean fields, I also recommend molybdenum as a seed treatment when the soil pH is 6.5 or lower, or during the first year that lime is applied. A recommended rate of molybdenum is 0.2 ounces per bushel of seed. This means that molybdenum-treated seed should be planted in most fields. Although molybdenum is only required in small amounts, it is essential to profitable soybean production. In university tests, soybeans treated with molybdenum yielded 12 to 15 bushels per acre more than untreated beans. Molybdenum is important to the nitrogen-fixing process, and if it is not available in adequate quantities, plants display a yellowish color typical of nitrogen deficiency. Although the use of molybdenum is important to growing soybeans, it should not be considered a substitute for limestone.

Research has demonstrated that a soybean plant requires good fertility but is not a heavy user of fertilizer. In most cases, the application of phosphate (P subscript 2O subscript 5) and potash (K subscript 2O) is all that will be needed. Soils that test medium to low in phosphorus (P) and potassium (K) should receive 40 to 80 pounds per acre each of phosphate and potash. If a soil test indicates soil levels of P and K to be high or very high, phosphate and potash applications can be omitted without reducing yields.

Proper liming is also one of the most important steps in a no-till corn fertilization program. Large amounts of nitrogen applied to the soil surface often produce extremely acidic conditions in the surface layer. If that soil is not neutralized, the acidic layer will reduce atrazine activity, decrease nutrient availability, increase concentrations of toxic manganese and retard root growth. Therefore, the pH of soil in continuous no-till corn should be checked annually for the first two or three years. After that, fields should be tested every two years.

Where lime is needed, apply the same amount of ground limestone that is recommended for conventional practices. Research does not indicate any advantage to applying smaller amounts more frequently. If soils are extremely acidic initially (pH approximately 5.0 or less to a 6-inch depth), use conventional tillage, apply recommended limestone and mix with the soil by disking or turning. The field can then be planted no-till the following year.

Lime and fertilizer should be applied according to soil test recommendations. Usually, you should not use less than 120 pounds of nitrogen per acre on no-till corn. For soils with the potential to make use of more than 120 pounds of nitrogen per acre, the rate for no-till should be the same as for a conventional tillage system. A good `rule of thumb' for Nitrogen rate is 1.2 pounds of Nitrogen per bushel of corn yield potential for a particular field or soil type. Where a good cover crop of vetch or crimson clover has been grown and terminated, decrease the nitrogen rate 50 to 70 pounds per acre.

Good timing and choosing the right application method are very important components of a successful fertilization program for no-till corn. Nitrogen should be applied just prior to or during planting, or it can be side-dressed before the corn is 12 inches tall. Split applications of nitrogen may be helpful when nitrogen rates are greater than 120 pounds per acre, or on soils that tend to be wet from poor drainage or high rainfall. Do not broadcast nitrogen solutions over the top of corn. If nitrogen solutions are used after corn is up and growing, use dribble tubes instead of fan nozzles to prevent leaf burn and to decrease the loss of urea due to volatilization.

Granular urea and ammonium nitrate can be broadcast over the top of corn. Leaf burn is greater with ammonium nitrate, but volatilization loss is greater with urea. To minimize corn leaf burn, broadcast the dry nitrogen materials when corn is less than 12 inches high and leaves are dry. Phosphate and potash should be applied to corn just prior to or at planting. However, if the soil pH is properly adjusted, P and K may be applied in the fall on land that is not subject to erosion or flooding. Lime should be broadcast on corn any time between harvest and planting, but fall application is better.

Corn makes efficient use of fertilizers applied broadcast on top of terminated sod or mulch-covered soils. The additional moisture usually associated with the mulched layer enhances nutrient uptake and encourages good root growth near the surface. Phosphate, potash and lime broadcast on the soil surface will not be lost unless soil erosion or flooding occurs.

All of the common sources of phosphate, potash and micronutrients may be used effectively in no-till corn systems. However, urea may not be as efficient as other nitrogen sources in no-till. The problem is ammonia volatilization losses resulting from the breakdown of urea during warm, rapidly drying conditions. Although urea is a good source of nitrogen for top-dressing cool-season grasses, nitrogen is usually applied to no-till corn later in the spring when temperatures are warmer and the risk of volatilization is greater. If the application of urea to no-till corn is followed by rain within three days, only small volatilization losses occur. Research has shown that placing urea or urea-containing fertilizer just below the soil surface with a coulter effectively reduces volatilization losses.

There are no "short cuts" to effective corn and soybean fertilization practices. Growers need to keep costs as low as possible while maintaining optimum yield levels. Increase your profit potential by taking inventory of the soil's chemical condition through soil testing and by applying only those fertilizer materials that will produce an economic benefit.