Soil Testing and Fertility Evaluation

by Jeffrey C. Silvertooth,
Extension Agronomist - Cotton

At this time of the year it is good for growers to review their records concerning soil test information for the fields on their farm. Soil samples should be taken at least every three seasons, unless specific problems are being experienced that would indicate any difficulties associated with fertility, salinity, sodium, or drainage problems which would require more frequent and detailed soil sampling. So if soil samples are needed, and have not been taken yet, there is still time before the start of the 1999 season to get this taken care of.

When collecting soil samples, soil cores should be taken from a depth of at least 6 inches. However, due to the irrigated nature of Arizona fields, it is recommend that the top 12 inches be sampled for most cases. Since it is important to develop an assessment of the residual NO₃^--N (nitrate -N) concentrations in the soil which can be available to the next crop, it is even more important to sample the top 12 inches and even the top 24 inches if at all possible.

An evaluation of soil conditions for phosphorus (P) should include a sodium bicarbonate (NaHCO₃) extraction. A soil P level of 5 ppm P or greater from the NaHCO₃ extraction would indicate sufficient P for Upland or Pima cotton (Table 1).

Recent work in the area of K fertility on cotton in Arizona, has shown most soils to have adequate K supplying power. However, there are circumstances in which a K deficiency may develop for cotton. For example, in cases where a cotton crop is following alfalfa, soils are coarse in texture, and have relatively low available K (less than 150 ppm K) as shown from an ammonium acetate (NH₄OAc) soil extract, some K fertilization may be in order.

We should also be careful to monitor soils for salt accumulations that can limit yield and be particularly damaging during crop establishment and during early stages of the season. This can begin to develop as a problem at a gradual rate, particularly when fields are not being rotated regularly. Soil salinity levels showing an electrical conductivity from soil extracts (EC_e) above 4 mmhos/cm would indicate possible problems. If salt problems are suspected for a given field, a separate set of soil samples within only the top 2-3 inches may be in order to evaluate the soil for conditions which may be damaging to developing seedlings. Similarly, soil sodium (Na) levels should be monitored by use of soil testing. Sodium adsorption ratios (SAR) should not be greater than 13. If a soil SAR is 13 or more, NA corrections should be considered with treatments such as gypsum, sulfur, sulfuric acid, etc., attempting to replace the NA with calcium (Ca). A soil may actually experience crusting and water penetration problems if the soil SAR > 8, particularly if the soil is fine textured. Therefore, it is important to evaluate fields that may be experiencing water infiltration problems based upon observations. Fields that have increasing problems with water infiltration and soil crusting, difficulties in developing and maintaining plant vigor, and/or diminishing yields; should consider soil samples to possibly diagnose saline and/or sodic soil conditions. It is also important to understand the difference between a saline and a sodic soil condition, both of which can be damaging, and each requires specific steps for correction.

Other important soil measurements should be evaluated to maintain good crop nutrition and soil conditions from a soil test. Some additional guidelines are provided in Table 1. Growers should not assume what the soil conditions are if they are interested in good yields and an efficient use of fertilizers.

A soil-testing program actually begins with the collection of soil samples from the fields in question. The objective is to use one sample to represent an entire field. Due to the natural variability of the soil in a field, it is important to collect a set of sub-samples from many parts of the field to form a composite sample that will reflect the true nutrient status of the field. It is usually recommended that at least 25 sub-samples should be collected in a field to provide a representative composite sample that is then sent to the laboratory for the desired analyses. As was previously mentioned with respect to salinity and/or sodicity evaluations, the depth of sampling can be an important factor to consider as well. For general soil analyses, a sample representing the top six to 12 inches of soil is usually collected. It is important to remember that the entire soil-testing program is really no more accurate than the accuracy of the soil sample in characterizing the nutrient status of a given field.

Table 1. Soil fertility and soil testing guidelines for Arizona cotton.

The guidelines provided in Table 1 are suggested critical levels for cotton production in Arizona. The critical levels should be considered as minimal soil testing levels, below which soils would be considered as deficient for that nutrient and a high probability should exist for a positive crop response to proper fertilization with that nutrient. These guidelines are under review at this time. A statewide project was initiated in 1997 to address the issue of soil test calibration for cotton production in Arizona. This project is being continued in 1999. Several experimental locations will be established among the cotton producing areas of Arizona to continue this calibration work. Studies will be placed on grower-cooperator fields in cooperation with several fertilizer companies across the state.

Full Disclaimers

Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, James A. Christenson, Director Cooperative Extension, College of Agriculture and Life Sciences, The University of Arizona.

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