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Soil sample now
by Chris Welsh, AFC Agronomist

If you have not already done so, now is the time to pull soil samples. The rainfall and high yields of this past growing season may have changed the soil fertility levels in your fields more than you think. 

Leaching, erosion and plant uptake were higher last summer than in the past several years, causing nutrients to be lost and pH levels to be lower. In order for a crop to reach its full potential, the soil that it is growing in must contain adequate levels of available nutrients. If levels of any essential element are inadequate, or if the pH of the soil is rendering them unavailable, top yields cannot be achieved.

Soil testing to assess the fertility level of a soil is basic to a successful soil fertility program. In order to adjust nutrient and pH levels to optimum levels for crop production, accurate soil samples should be taken. Soil samples that are taken improperly may not represent the best average fertility level for a field and may lead to poor nutrient management.

Just what exactly is an accurate sample? Accurate here is a relative term. There are a lot of opportunities for error when getting soil test data for a field, but the following ideas can help to keep your information as correct as possible.

Depth of sampling

Soil sampling depth is crucial for accurate representation of what is available in soil for plant uptake. The idea here is to examine nutrient levels in the volume of soil that will be supplying the bulk of the nutrients. The depth that is sampled depends on crop and tillage practices being used. Placing a mark on the soil probe at the desired depth helps to insure that all cores are taken at the same depth.

In conventional tillage, samples should be taken to the depth of the plow layer, usually about 6 to 8 inches. Reduced tillage and no-till fields should be sampled to about 4 inches. Pastures should also be sampled at 4 inches. However, deeper samples to the depth of rooting of the grass may be helpful in determining levels of nutrients at deeper levels.

Consistency is important here. In order to track the effectiveness of your fertility program, you should try to sample to the same depth each year.

Number of cores

Usually 15 to 20 cores per sample are adequate. However, this amount may need to be increased in some cases. For example, when a pasture has been plowed for the first time in several years, as in peanuts following bahiagrass pasture, more cores should be pulled to overcome the effects of incomplete mixing of soil, plant material and manure. Also, if the previous crop was sidedressed with fertilizer (or had fertilizer banded at planting), and the field has been plowed, then the number of cores should be increased to lessen the effect of any bands that may be sampled.

Size of sample area

The smaller the area the more accurate the results, but the sampled area should be no smaller than can be treated individually. About 20 acres is an acceptable goal for each sample in fields that are uniform in appearance, soil type and past treatment. Areas that are visibly different should be treated separately.

Timing of sampling

The primary point to remember in intensively worked fields is to sample during the same season every year in order to better watch trends in a fertility program. It is probably best to sample during the fall for spring planted crops and in the spring for fall planted crops. It also helps to get soil sample results in the fall for spring crops so that lime can be applied early enough to raise low pH areas in the fields.

Sampling pattern over the field

Typically, taking cores at random in a zigzag pattern over the field provides a good representation of a uniform field. If the field has not been plowed, then care should be taken to avoid areas where fertilizer was sidedressed or banded. Other places to avoid in a field include fertilizer and lime dumpsites and areas where livestock have congregated.

Handling of sample

Cores should be placed into a plastic bucket as they are being pulled. Galvanized buckets may contaminate samples if micronutrients are being tested. The cores should be thoroughly mixed in the bucket, and an adequate portion should be placed into the soil sample bag or box. Most sample bags have a fill line marked on them, but if not, about a cup of soil will be adequate.

Complete records should be kept, including field names, cropping history, when sampled and when and how fertilized. This information along with the sample results will help you make sound soil fertility decisions.



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Date Last Updated January, 2006