Agriculture News: Soil sampling, fertilizing in the fall a good start for spring

By Keenan Bishop, Published:

When lime and fertilizers are applied to soils, many chemical reactions take place — some immediately, and some over long periods of time. These reactions have a great influence on application timing and how efficiently fertilizer is taken up by growing crops, thus influencing the economic returns of fertilizer use.

According to UK publication AGR-5, nearly half of all the fertilizer used in Kentucky each year is bought by farmers during the rush of the spring planting season. Much of the fertilizer applied during the spring rush could be applied during other seasons of the year.

If farmers scheduled fertilizer applications more evenly throughout the year, discounts may be available, custom applicators will have more time and much of the fertilizer necessary for spring planting would already be on the field. This saves valuable time during spring planting and eliminates the risk of not being able to apply fertilizer because of wet land in the spring. 

This is why we encourage soil sampling in the late summer and fall with free soil tests in Franklin County (maximum of five) through this month. Another reason is the soil lab is less busy now than in the spring when it can take several more weeks to get results back. Bring your samples in by Thursday to take advantage of the free samples, quicker turnaround and get a jump start on spring.

Apply lime now

Lime can be applied any time of the year and the rule of thumb is the sooner the better. When applied to soil, the liming material reacts with soil. The rate at which the lime material dissolves is largely controlled by how finely it is ground and the chemical form of the material. 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. 

Lime particles are attracted to and are held by clay particles in soils after it 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, there is no concern in Kentucky over leaching losses of lime. For this reason, lime can be applied whenever practical, although it should be applied far enough before planting to allow time for adequately reacting with the soil and neutralizing acidity. 

Nitrogen (or N, the first “0” in 0-0-0 on the fertilizer bag) can be used most efficiently by a crop if it is applied either while the crop is growing or just before it starts to grow. Even with these 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 that tend to waterlog. 

Phosphate (P) fertilizer (the second “0”), 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, which is a major source of pollution of our streams. 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. 

Potassium or K (the last “0”) dissolves from fertilizer in the soil, is attracted to clay particles, and is then held tightly enough that leaching losses are negligible. 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. There are always exceptions to the rule and this is the case with alfalfa, a known “accumulator.” Alfalfa likes to take up way more K than it can use so we recommend applications after the first cutting. 

The only drawback to fall sampling is seasonal fluctuations, but if you sample the same time every year you’ll be consistent. The pH is usually lower during the summer and early fall and then increases as the soil moisture increases.

 Potassium fluctuations are seen almost yearly and can be large. As the crop grows and matures, uptake lowers the soil-available K. For crops where the entire plant is harvested such as tobacco, hay and silage, K removal is highest since everything taken up by the plant is removed from the field.

P fluctuations are smaller than for pH and K. The quantity of crop P uptake is much lower than K. For specific details on these fluctuations for different crops ask for AGR-189 or visit http://www2.ca.uky.edu/agc/pubs/agr/agr189/agr189.pdf

More frost?

A little late since most of us have had a frost and even a killing freeze probably (I wrote this Tuesday) but if you’re wondering what the chances of a frost are for the following week visit weather.uky.edu/nx/n_min_ky.shtml.

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