SOIL SCIENCE NEWS AND VIEWS Vol. 17, No. 2, 1996
FERTILITY STATUS AND CHANGES OF 50 CRP FIELDSThe CRP program in Kentucky has 435,000 acres of land which will be released soon. Since these fields were highly eroded when they were placed into the program, it was felt it would be helpful to determine the general fertility status of the CRP fields in Kentucky and see what changes had taken place during their years of enrollment in the CRP program. Since most of the fields had a previous erosive history, we expected the fields to be low in pH and in the low to medium range for phosphorus and potassium. We would like to thank the producers who let us sample their fields and the county agents who helped coordinate the effort. METHOD In the spring of 1995, 50 fields which had been in the CRP program since 1986, 87, or 88 were surveyed for weeds, nematodes, insects and nutrient status. The fields (5 per county) were located in 10 different counties in western and central Kentucky. The counties were selected to reflect the geographical part of the state with significant CRP acreage and the fields within each county were selected to represent the array of soil types and past management of fields enrolled in the CRP program. An individual soil sample and analysis was made for each 10 acres of each field. Between 5 to 10 sites were sampled in each field. The individual analyses were averaged to obtain the field results. All sample cores were taken to a depth of 6 inches. This report addresses the organic matter, pH, phosphorus (P) and potassium (K) status of these fields. Soil tests were required for enrollment into the CRP program on most fields. In 35 of the fields this information was available and it was compared to the 1995 results in those 35 fields. Depth of soil sampling and method of sampling may have varied between the two samplings. Organic matter was not tested on the soil tests made at enrollment. RESULTS The results (as one might expect) are variable from field to field and there was significant variability among the individual sites in many of the fields. Some trends were found in many of the fields. The fertility status of the fields was better than expected with the exception of phosphorus. In the 35 fields where the 1995 soil tests could be compared with the soil tests at enrollment, it appeared that the pH and K soil tests were basically stable while the P soil test decreased. Organic Matter (O.M.) The organic matter in the fields was high in most fields (Table 1). The current average of the O.M. content is 2.3%. The fields (36%) that had the lowest O.M. content (1.5 to 2.0%) usually had a poor fescue stand. Based on previous studies and samples collected from eroded, continuously tilled fields, the O.M. was probably about 1.5% prior to enrollment into the CRP program. A few of the fields had very high O.M. %. The method of soil sampling may have had some effect on the results. Table 1. Organic Matter (%) in Surveyed Fields _________________________________________________________________ O.M. Content % of fields ----------------------------------------------------------------- 1.5-2.0% 36 2.1-2.5% 40 2.6-3.0% 16 > 3% 8 _________________________________________________________________ Range 1.5 to 3.8% Average of all fields 2.3% _________________________________________________________________ pH The soil pH's appear to be in the favorable range for most crops grown in Kentucky. It was expected that many fields would have a low pH. However, almost 80% of the fields had a pH between 6.0 and 7.0. Only 8% of the fields had a pH below 6.0. This indicates that the amount of lime required to place these fields into crop production will not be great. In the 35 fields where the soil pH could be compared with that at enrollment, the pH's were well maintained throughout the seven to nine year period. The fields testing in the low range decreased from 29% to 6% which may indicate that lime was added at seeding for CRP purposes on some of these fields. The accumulating organic matter at the soil surface seemed to have little effect on the pH. The addition of no nitrogen is probably the key to the stable pH. Table 2. Soil pH of Surveyed CRP Fields* _________________________________________________________________ pH Range 50 Fields (%) 35 Fields (%) Before 1995 _________________________________________________________________ < 6.0 8 29 6 6.0-6.5 60 29 54 6.6-7.0 28 31 37 > 7.0 4 11 3 _________________________________________________________________ Range - 5.3 to 7.5 _________________________________________________________________ * Soil pH in the 50 fields surveyed in 1995 and a comparison with the pH prior to CRP enrollment on 35 of the fields. _________________________________________________________________ Phosphorus (P) There was a wide range in the amount of P found in the fields. Most of the fields (62%) were in the low range and 90% were found to be in either the low or medium range. This means that most of the fields will need to receive a substantial amount of P fertilizer for optimum production. This appears to be the most limiting nutrient in most of these fields. The reduction of the P soil test may be due to several reasons such as: P reaction with the soil over the years or stratification of P at the surface due to recycling and lack of physical mixing by a tillage operation. When P soil tests in 35 comparative fields are examined, a large decrease is seen. The percent of fields testing in the low range went from 14% to 57%. The change in the high range was just as dramatic. These soils are naturally very low in available P and since no P has been added for several years, the P will slowly move to a more unavailable form. Some of the P may also be in the organic form. This would be contained in the increased organic matter at the surface and would not be extracted or measured by the standard soil test procedure. This organic P will be available to plants as the organic matter is decomposed. Table 3. Phosphorus Soil Test in Surveyed Fields* _________________________________________________________________ Range (lb/ac) 50 Fields (%) 35 fields (%) Before 1995 _________________________________________________________________ Low (0 to 30) 62 14 57 Medium (31-59) 28 26 31 High (60+) 10 60 11 _________________________________________________________________ Range 3 to 200+ (lb/ac) _________________________________________________________________ * Phosphorus soil test in the 50 fields surveyed in 1995 and a comparison with the phosphorus soil test prior to CRP enrollment on 35 of the fields. _________________________________________________________________ Potassium (K) The K content of the fields was higher than one might expect. Only 10% were in the low category. Although 58% (low and medium range) will need to receive some K fertilizer, 42% will require none the first year of production. This is probably due to K being deposited at the soil surface by the growing plants over the years without any removal. The results of the K soil test on the 35 comparative fields indicate that the values were very similar or slightly increased over the years. The K content in the top few inches of soil would be expected to increase since the plant roots remove K from the subsoil and place it at the surface in the residue. As long as the above ground part of the plant is not removed, this trend should continue. The total K content of the plants is much higher than the P content which may explain some of the differences between the two elements. Table 4. Potassium Soil Test in Surveyed Fields* _________________________________________________________________ Range (lb/ac) 50 Fields (%) 35 fields (%) Before 1995 _________________________________________________________________ Low (0 to 199) 10 8 3 Medium (200-299) 48 26 46 High (300+) 42 66 51 _________________________________________________________________ Range 139 - 500+ (lb/ac) _________________________________________________________________ * Potassium soil test in the 50 fields surveyed in 1995 and a comparison with the potassium soil test prior to CRP enrollment on 35 of the fields. _________________________________________________________________ Conclusions The survey indicates that the fields in the CRP program had a reasonably good fertility status when placed into the program and that changes since then have not been great. The soil pH has been maintained under these conditions and the need for lime will not be great for most fields. The P is low on most fields and has decreased over the time of the program. This will be one of the most limiting nutrients on most fields and will require use of a significant amount of P fertilizer for best production. The K content on most fields is medium or high and most fields will require none or only moderate amounts of K fertilizer for production purposes. There are high amounts of variability between fields, so each field must be tested and treated separately to assure adequate fertilization and liming for good production.
Lloyd Murdock and Dottie Call