Nearly half of all the fertilizer used in Kentucky each year is bought by farmers during the rush of the spring planting season. As a result, it is often difficult for the fertilizer industry to meet customer demands during this 6 to 8 week period. Much of the fertilizer now used during the spring rush season could be applied during other seasons of the year. If farmers scheduled fertilizer applications more evenly throughout the year, dealers would be in a much better position to provide them with the kinds and amounts of fertilizer they desire. In many instances, off-season discounts could be obtained. A more even applications throughout the year would mean that much of the fertilizer necessary for spring planting would already be on the field. This would save valuable time during spring planting and eliminate the risk of not being able to apply fertilizer because of wet land in the spring.
Fertilizing in the fall would also ease problems of soil compaction caused by spreading equipment since the soil would have time to rest over winter. Chemical reactions take place when lime and fertilizer are applied to soils, some immediately, and some over long periods of time. These reactions have a great influence on when lime and fertilizer can be applied and how efficiently fertilizer is taken up by growing crops, and this influences the economic returns from lime and fertilizer use.
The amount of surface area exposed in soil with which lime and fertilizer come in contact is extremely important. This factor is related to the amount of clay present in the soil, since most of the soil’s potentially reactive surface area is made of clay particles and since, per given volume, there is more surface area in clay than in sand or silt. The mineral form of individual soil particles and the extent to which these mineral soil particles have been coated with layers of oxides and organic materials also affect the total soil reaction. All clay particles in soil have a net negative (-) electrical charge. Because of this negative charge, clay particles will react with components of lime and fertilizer which dissolve as positively (+) charged particles (cations) when added to soil. Lime materials dissolve to release positively charged calcium and/or magnesium particles, and some fertilizers dissolve to form positively charged particles (ammonium nitrogen (NH4+) and potassium (K+). Such positively charged particles (cations) are attracted to the negatively charged components of soils.
Materials commonly used as lime are either ground-up limestone rock, ground-up marl or products of limestone which have been altered by burning to make them more water soluble than the ground-up rock itself. By far the most common liming material in Kentucky is ground limestone (aglime), most of which is calcium carbonate. When applied to soil, the liming material reacts with soil moisture to release particles of calcium. The rate at which the lime material dissolves to release these particles is largely controlled by how finely it is ground and the chemical form of the material (carbonate, oxide or hydroxide). The finer the material, the more rapidly it dissolves. Oxides (burned lime) and hydroxides (hydrated lime) are more soluble in water and react much more quickly than carbonate forms of lime (calcitic aglime or dolomitic aglime). Calcium particles are attracted to and are held by clay particles in soils after the liming material dissolves. This neutralizes soil acidity, increasing soil pH readings. It also means that lime does not move very fast downward through soil. With the exception of extremely sandy soils, although it should be applied far enough before planting to allow time for adequately reacting with the soil and neutralizing acidity.
It is important to have enough available nitrogen in the soil at the time when the crop’s growth is most rapid. Basically, we use two types of nitrogen: 1) ammonium or ammonium formers (urea, anhydrous ammonia); and 2) nitrate forms. In aerated soils, ammonium nitrogen is changed in the soil to the nitrate form. The nitrate form is completely soluble in soil water and therefore moves in the direction of soil water movement. Soil temperature influences the rate at which ammonium nitrogen is changed to nitrate. It is generally considered that the rate at which this change takes place is negligible at freezing or below, while at soil temperatures above 50°F, the conversion rate is rapid. In Kentucky, soil temperatures during the winter generally do not stay cold enough to completely prevent conversion of ammonium nitrogen to nitrate nitrogen. This means that fall or winter application of urea, or other ammonium forms of nitrogen fertilizers to fallow land is not a sound practice.
Since nitrate nitrogen moves with soil water, it is subject to loss from the plant rooting zone by leaching during periods of high rainfall. An excess of water usually occurs in Kentucky during the months of December to April. Since nitrate contains oxygen in its structure, losses of nitrogen fertilizer from water-logged soils can be quite severe. Even with precautions, there will be occasions when split applications of nitrogen should be helpful. For example, heavy rains following applications of N can result in leaching losses from permeable soils or in denitrification losses in soils which tend to waterlog.
Phosphate fertilizer, when applied to soil, reacts very rapidly to form compounds that are less soluble than the form in which the fertilizer was added. For this reason, phosphates are not mobile in soil, and leaching losses of phosphate are insignificant in Kentucky soils. Erosion of surface soil containing applied phosphates is the most likely way in which phosphates would be lost from soils. Since phosphates are relatively immobile in soil, it is not important that phosphate fertilizer be applied during the crop’s growth cycle. The long residual value of phosphates in soil will make them available to crops over long periods of time. It is rare that more than 25 percent of phosphates applied to a crop is used in the first year after application.
Potassium dissolves from fertilizer in the soil, is attracted to clay particles, and is then held tightly enough that leaching losses are low. In sandy soils which have very little clay, leaching losses of potassium can be a problem. This is not of general concern in Kentucky, however, since only a small portion of agricultural land in Kentucky is of sandy texture. In most Kentucky soils, potassium is not likely to leach, and there should be no great practical concern over when it is applied to the soil. The most likely losses of potassium would be through sediment losses from erosion. It is unlikely that more than 40 percent of potassium applied in a given year would be used by crops during that year.
It is quite evident that much of the fertilizer added to Kentucky soils does not have to be applied during the spring planting season to be effective in soil improvement. Much of the fertilizer needed for profitable crop production can be applied during other seasons of the year without sacrificing yields. A shift away from traditional spring applications of fertilizer can help save valuable time and assure better service from the fertilizer industry.
For more information contact the Pulaski County Extension Office. Article content taken from UK publication “AGR 5”When to Apply lime and fertilizer By Kenneth Wells