Trends from soil samples show what's missing from our soil and how it can be improved for increased crop-growing efficiency.

Soil testing is not a new concept and is still the best basis for a fertilizer recommendation. A properly taken soil test can result in appropriate recommended rates of fertilizer and limestone. However, a poorly taken soil test can result in under or over application of fertilizer and limestone.

Let's look at 8 years worth of composite samples taken by MFA that were run either through the University of Missouri soil-test lab or Midwest Labs for pH, phosphorus and potassium results. Limitations in these results include: Only samples from two labs were included in this summary; we could not separate out any lawn and garden samples or samples taken when trying to diagnose field problems, and finally, we could not take out samples from the same field taken in different years. Then we will compare the MFA results with soil test summaries taken by the Potash and Phosphate Institute (PPI) for Missouri in the years 1986, 1993, 1997 and 2001.

Table 1 shows the salt pH results for 8 years worth of MFA composite samples. For optimum row-crop production target pHs should be 6.1 to 6.5, while forage grasses targeted pHs would be 5.5 to 6.0. In looking at the results, 61 percent of the samples would not be in the optimum range for row-crop production and 30 percent of the samples would not even be in optimum range for cool-season grass production.

Table 1: 8 Years of MFA composite sample results for soil pH

Table 2 shows the PPI trends for Missouri soil pH. These numbers are reported as water pH values. A water pH value of 6.0 would be similar to the salt pH of 5.5. As can be seen by these results approximately 35 percent of the soil tests in Missouri are less than or equal to a 6.0 pH in the 2001 sample but also for the 1993 and 1986 sample years. In reality the soil pH results in the PPI study were very consistent over the years the study was conducted.

Table 2: PPI summary for Missouri sample results for soil pH

It is interesting to note that while the MFA numbers are for 8 years worth of sampling, and the PPI numbers are for individual years, the trend for soil pH is similar. The end result means that there are plenty of fields that need additional limestone applied to raise soil pH to optimum levels for crop production.

Table 3 shows Bray I phosphorus results for 8 years worth of MFA composite samples. For optimum row-crop production and alfalfa production, target Bray I phosphorus levels are 45 to 55 pounds per acre and 40 to 50 pounds per acre for forage grasses and clovers. The results show approximately 15 percent of the samples fell in the optimum range while greater than 60 percent of the samples fell less than or equal to 40 pounds per acre. This means that 60 percent of the samples would require not just fertilizer application for grain or crop removal but additional phosphorus to raise the soil test to an optimum level. Over 24 percent of the samples tested above the optimum range (either no phosphorus fertilizer would be required or only a slight amount). It is interesting to note that of this 24 percent of samples testing above the optimum range, the average Bray I phosphorus level is 100 pounds per acre, which could be conducive to heavy manure sites or river bottom soils naturally high in soil phosphorus.

Table 3: 8 years of MFA composite sample results for soil phosphorus

Table 4 shows the PPI trends for Missouri soil phosphorus. As can be seen by these results, in the 2001 study, approximately 69 percent of the soil tests in Missouri are at a medium level of soil phosphorus or less. Individual numbers in the years of the PPI study varied, but on average, it still resulted in 58 percent of samples testing at a medium level of soil phosphorus.

Table 4: PPI summary for Missouri sample results for soil phosphorus

Just like soil pH results, soil phosphorus results indicate that to raise the soil phosphorus to optimum levels for crop production, about 60 percent of the tested soils need additional phosphorus.

Table 5 shows the results of potassium for 8 years worth of MFA composite samples. For optimum row-crop production and alfalfa production, target potassium levels, depending on soil type, are 250 to 350 pounds per acre. For forage grasses and clover, the range is 180 to 230 pounds per acre. Results show approximately 1/3 of the samples fell in the optimum range for row crops and alfalfa; 1/3 of the samples fell in the area of less than or equal to 230 pounds per acre. The remaining 1/3 of the samples fell in an area greater than or equal to 351 pounds per acre. This means that approximately 1/3 of the samples would require not only a fertilizer recommendation for grain or crop removal but additional potassium to raise the soil test to an optimum level. Approximately 1/3 of the samples tested above the optimum range--either no potassium fertilizer would be required or only a slight amount. Interestingly, of this 1/3 of the samples, the average potassium level is 507 pounds per acre. This could be a result of some natural, very high-testing soils in MFA's trade territory.

Table 5: 8 years of MFA composite sample results for soil potassium

Table 6 shows the PPI trends for Missouri soil potassium. As these results show, approximately 52 percent of the soil tests in Missouri are at a medium level of soil potassium or less for 2001. The PPI numbers do show a definite decline of samples testing at medium or less in soil potassium (from 76 percent in 1986 to 52 percent in 2001).

Table 6: PPI summary for Missouri sample results for soil potassium

Unlike the soil pH and the soil phosphorus results, the MFA numbers and PPI numbers for potassium vary from 33 percent for MFA to 52 percent for PPI in 2001. We really cannot explain the trend difference here, but the end result means that at least 33 percent of the fields tested need additional potassium to raise the soil potassium level to optimum levels for crop production.

What do all of these results mean? They reinforce the need to get accurate soil-test information to ensure the correct amounts of fertilizer get applied. Every year problems arise in fields because of fertility problems. Proper soil testing could help spot these problems before they start robbing yield. n