Building Up and Drawing Down Soil P and K Levels

It is common to hear people discuss fertilizer recommendations in terms of crop removal, plus or minus soil test buildup as if they were somehow disconnected from each other. In fact, they are two aspects of the same subject. Most of us fall into the habit of thinking that we are fertilizing plants. Except for foliar fertilizer or tree trunk injection, we do not fertilize plants… we fertilize soil. Because of this, soil chemistry will determine how much of the applied nutrients the plants will be able to take up. If a soil is low in phosphorus (P) or potassium (K), it will tie-up or “fix” much of the applied fertilizer P and K (P₂O₅ and K₂O) into forms that are not available to the plants. This nutrient fixation is simply another way of saying that the soil is trying to build itself up in these nutrients… whether that is your intention or not. The soil is a reservoir for the nutrients that have been applied or generated by other means over the years. The nutrients that a plant gets in any one season are likely to be ones that have been in the soil for many years. Therefore you can think of fertilization as putting nutrients into one side of a reservoir while the plants are taking them out of another end. What happens inside of this nutrient reservoir is soil chemistry and microbiology. These processes, along with weather, determine how much access the plants have to the nutrients within the reservoir.

A soil with a weak P level and a medium-to-strong P fixation capacity will convert or “fix” much of the applied fertilizer P into unavailable forms and leave little for the crop. In many medium P soils as much as 65% of the applied fertilizer P can be fixed. In extremely poor soils, as much as 90% of applied P may be fixed. In these cases, applying only “crop removal” is inviting P deficiency. In such soils the P fixation capacity must be overcome by higher fertilizer P rates in order to have enough P left over for the crop. In other words, soils with less than optimum P levels must have some soil buildup P, to insure that there is enough for the immediate needs of the crop or other plants. The difficulty lies in determining, with some accuracy, the minimum amount of buildup P required to overcome a particular soils fixation capacity during the season.

Soil test buildup application rates are typically discussed in terms of the ratio of applied P₂O₅ to soil test P buildup amounts. Often you will hear that most soils require about 9 or 10 lb. of P₂O₅, in excess of crop needs, in order to increase the soil test P by 1 lb/ac. In ppm this would be 18 to 20 lbs of P₂O₅ to raise the soil P 1 ppm. Agronomists normally refer to this as the buildup ratio. A buildup ratio of 9 lb. P₂O₅ to build up the soil test by 1 lb/ac soil test P would be simplified to 9:1 or sometimes just the number 9. In ppm this same ratio would be stated as 18:1 or just 18.

If you have heard agronomists quote this 9:1 or 18:1 ratio many times you might think that this is all that you need to know… it isn't that simple. Research at the University of Kentucky reported in 2002 (Table 1) illustrates how soil test P buildup occurs. In this study, they looked at 16 different soils with initial soil test P levels ranging from 6 to 240 lb P/acre (3 to 120 ppm P/a) using Mehlich 3 (M3) extraction. They applied 6 different rates of phosphorus ranging from 38 to 228 lb P₂O₅/acre. Their results nicely fit a mathematical relationship used to develop the data in Table 1. These results clearly show that the initial soil P test is a major factor in determining how much P₂O₅ is required to buildup the soil test P.

While other soils in different locations may have somewhat different results, the general trend will probably be similar. Notice that when the soil P test is very low, the soil P buildup ratio is very high. However, as the initial soil P level increases, the buildup ratio gets lower. You can see by this data that the most common soil P levels found in farm fields that need some buildup have a buildup ratio of approximately 9:1 or maybe a little lower.

Work by Kamprath in 1964 (Table 2) illustrates that higher individual rates of application may also affect the efficiency of buildup. While we would expect higher annual rates of P₂O₅ to result in a higher soil test, this work indicates that the higher annual application rates may also be more efficient (a lower buildup ratio) at raising soil test P levels over time. In other words, with higher annual rates, it may take less total fertilizer P to reach the ultimate soil test goal.

Soil test P drawdown works similar to buildup, but in reverse. Also like buildup, there is no single number or calculation that will work in all situations.

Another University of Kentucky study (Table 3) looked at the effect of growing alfalfa for several years without fertilizer on soil test P (STP) drawdown.

Note: the data on each of the Low and High lines represent the respective values from all of the 16 plots should not be viewed as representing a single plot in any case. Wells & Dollarhide, 2002. Mehlich-3 extraction

In this study, 16 sites on a single farm received no fertilizer between May 26, 1998 and Feb 23, 2000. During this time the existing alfalfa was harvested in the normal manner and analyzed for P removal. The researchers reported that average P₂O₅ removal was 13.9 lb P₂O₅/T. An average of 65 lb P/ac (150 lb P₂O₅) was removed from the plots and the STP was reduced an average of 19 lb/ac for an average P₂O₅:P drawdown ratio of 11.8 to 1. The range of total P₂O₅ removal was from 128 to 169 lb/acre and the average STP drawdown ranged from 4 to 44 lb P/acre for a range in drawdown ratios of from 3.2:1 to 37.8:1. This data did not fit into a simple equation which could generate a simplified table or rule-of-thumb for us to use, so producers are advised to simply monitor their soils with routine soil tests in order to properly manage their soil fertility. While there was some evidence of lower initial STP having a higher P₂O₅:STP drawdown ratio, the relationship was not as clear as seen with soil test P buildup.

Figure 1

In one study (Fig 1), the researchers made different single applications of P₂O₅ then grew wheat without additional P₂O₅ for several years to measure the effect on soil test P. It took between 6 and 9 years for the soil test to reach a “base-line” P level. As Fig. 1 shows, the higher rates of P₂O₅ resulted in somewhat higher new base-line soil test P levels.

Figure 2

Another study in North Carolina (Fig 2) looked at soil P test drawdown over a longer time period. The way in which the investigators applied fertilizer P is a little complicated, so you might want to read it twice. They applied P₂O₅ by two methods. A single application of 289 lb P/ac (662 lb P₂O₅/ac) was compared to 8 annual banding applications prior to the beginning of the study at a rate of 54 lb P/ac each (a total of 989 lb P₂O₅). No additional P was applied for 26 years, while growing corn and soybeans. As seen in Fig. 2, it required between roughly 17 and 23 years to reduce the elevated soil P to their “critical “soil P level of 44 lb P/ac. Unfortunately, the report did not document the annual yields of corn and soybeans that caused the P test drawdown. However, the nature of both curves in Fig. 2 shows that the soil test drawdown was a little faster in the initial years when the soil test was higher, than at lower levels in later years. In other words, as a soil test level approaches its “base-line” it is more difficult to change it.

From this information we can conclude the following points

1. The P₂O₅/P buildup ratio for any soil will be higher with lower initial soil test P levels.
2. Higher individual application rates of P₂O₅ are likely to be more efficient at increasing soil P levels than multiple smaller ones, even at the same total application rates.
3. Soil test P drawdown varies by soil type, fertilizer management history, and will work somewhat like buildup in reverse.

Setting soil test P goals can be more complicated that it may seem at first. Many factors should be considered. For example, different crops normally require different soil P levels for best yields. For example, wheat requires a much higher soil P test than corn, while soybeans seem to grow well at a much lower soil P test than corn. At first glance, you could conclude that each crop should have a different soil P goal. However, a field can have only one soil P level, so which one do you choose if the three crops occur in rotation? Some producers of acid-requiring crops like potatoes have another problem. An acid soil pH causes much more soil P tie-up than a pH between 6.0 and 7.0. In these situations, a higher than “normal” soil P level is often beneficial.

Over the years some researchers have noticed that the “ideal” soil P test appears to decrease as the soil CEC increases. While this does not appear to be a large effect, and it is not widely used, Spectrum Analytic has incorporated it into our recommendation program. The information (Table 4) describes our method of defining a desirable soil P test for most crop rotations and situations.

While the chemistry that controls soil P and K is much different, the ultimate effects of building or drawing down the soil test of either have similar patterns.

With soil test K, most agronomists use a K buildup rate of 3 to 5 1b./acre K₂O: 1 lb./acre soil K (6 or 10 lb./acre K₂O:1 ppm soil K). However, research indicates that, like P it depends somewhat on what the initial soil test is, plus other factors. Research at the University of Kentucky (Table 5) looked at this question in 1990 and found the following buildup ratios for one soil.

The data in Table 5 is taken from a single soil type; therefore variables such as the different types and amounts clay are not much of a factor. The same experiment performed in other soil types would likely show somewhat different results, but would likely have similar patterns. It would be reasonable to assume that soil test K drawdown would have patterns and ratios similar to those of buildup.

A South African study (Johnston et al, 1999) looked at the effects of the soil physical properties on soil test K buildup in 51 different soils. Unfortunately, they did not include the initial soil K test level in this study. However, their results do shed light on some other aspects of soil K buildup. They found that on average it required 3.07 lb K₂O to raise the soil test 1 lb of K. However, the range of needed K₂O was from a low of 1.72 to a high of 10.51. Of the various soil physical factors evaluated they found the most significant factors causing a higher buildup ratio were the overall soil CEC, the CEC of only the clay fraction of the soil, a mathematical value called the K desorption index, and a mathematical factor relating the density of the soil to the CEC of the soil. While none of these factors was dominant, the relationships showed that higher CEC soils, and those dominated by certain types of clay required higher rates of K₂O to increase the soil a given amount. The researchers found, as had previous work, that mica and vermiculite type clay greatly increased the amount of fertilizer K₂O required to increase the soil test K. Next down on the scale were other 2:1 type clays and highly weathered 1:1 type clays had the least effects. While these different types of clay can occur nearly everywhere, Northern soils tend to be dominated with the more complex, K-fixing types and Southern soils tend to have more of the highly weathered, less complex 1:1 type clay which is less K-fixing.

Examples of the variability in the K fixation capability of different soil types is illustrated by some results published from work at Purdue and Michigan State University (Tables 6 & 7).

The wide range in the K-fixation capacity of soils within a single state illustrates why no single buildup factor will be correct in all situations. Unfortunately, this data did not include some of the information that might make the data even more valuable, such as initial soil K test or in Michigan data, the rates of K₂O application. However, from this you can appreciate that if 30% of the applied K₂O is fixed into an unavailable form and crop removal must be subtracted from the remaining 70%, the amount left-over to increase the available soil test K may not be very large. If your soil has a K-fixation capacity in the 60% to 90% range, you can imagine the difficulty in producing good crops or increasing the available soil K.

The University of Kentucky study that looked at soil test P drawdown (Table 3) also looked at soil test K (STK) drawdown (table 8). As with P, they looked at the effect of growing alfalfa for several years without fertilizer. A total of 10.8 tons/a was harvested over the two years with a significant droughty period in each year. An average of 51.8 lb K₂O/ton was removed in the harvest.

Table 8

Mehlich-3 extraction

As table 8 shows, the ratio of K₂O removed to STK drawdown averaged 3.7 (3.7:1), with a range of from 1.4 to 8.1. These ratios are similar to those found when building up soil test K.

Unfortunately the researchers did not report the CEC of the soils in the study. Missing the CEC meant that we could not evaluate any relationship between soil CEC or K saturation and the soil test drawdown. However, there did seem to be a relationship between the initial soil test K in simple lb/ac and the drawdown ratio. This relationship indicated that the drawdown ratio would tend to be higher with a lower initial soil test K. This would seem to be logical since as a soil test approaches its “natural” base level, it becomes increasingly difficult to decrease the soil test K further. Typically, the “natural” base or minimum soil test K level will be lower on a low CEC soil and higher as CEC increases.

The following data (table 9) lists some soil CEC and soil K levels used by Spectrum Analytic. While agronomists may differ, most would consider these to be adequate-to-high soil K goals. Using the previous information, you can estimate the amount of K₂O required to achieve these soil K goals. Don't forget that soil test buildup amounts occur only after accounting for crop removal.

A few very simple but helpful conclusions can be drawn from this information.

• Producers really don't have a simple choice of fertilizing for removal only, or removal plus buildup when the soil test is weak. The soil is going to try to build itself up whether we like it or not.
• Increasing the soil P or K test when either is very low will require more fertilizer input per unit of soil test increase than required if the initial soil test is higher.
• Soil test levels are reduced in about the same ways and efficiencies as they are built up.

1. The Nature and Value of Residual Soil Fertility, W.O. Thom, U of Kentucky, AGR-144, 1990
2. Drawdown of Soil Test Phosphorus and Potassium Levels by Alfalfa, K.L. Wells & J.E. Dollarhide, Univ. of Kentucky, Soil Science News & Views, Vol. 21, No. 3, 2000
3. Quantities of Potassium Fertilizer Required to Raise Soil Test Value. M. A. Johnston, et al, Univ. of Natal, RSA, Commun. Soil Sci. Plant Anal., 30(17&19), 2485-2497 (1999).
4. Optimum Soil Fertility Levels for Corn and Soybeans. Kamprath, E.J., Plant Food Review, 10(3):4-6, 1964