Recently, the question has been raised of whether or not agriculture should be using potassium (K) fertilizers. Let’s examine how soil fertility and plant nutrition scientists have determined if or when K should be applied.
It starts with the plant. Plants require 17 nutrients to develop properly. Potassium is one of these and is taken up in large quantities. It is therefore termed a “macronutrient.” Plants get their K from the soil via their roots. Consequently, one of the most basic questions that soil fertility and plant nutrition scientists have addressed over the past several decades is, “How much of a plant’s nutrient needs can be met by what’s already in the soil?”
To determine if a soil already has enough K, scientists apply incremental amounts of K, then measure the degree to which plants respond. A zero rate of K, termed a “check,” provides a basis for comparison. Increases in growth and yield with K additions, when compared to the check, indicate that the soil supply alone is not sufficient to meet the plant’s requirements.
An experimental design that is often used to measure response is the “omission plot.” Omission plots are sets of treatments that examine how the lack of one nutrient affects yields and nutrient uptake when all other nutrients are at sufficient levels.
Plant response has been and continues to be the basis for determining whether K is needed. One general type of approach, termed “plant-based” in this article, relies primarily on these types of plant measurements. The other approach, “soil-testing based” also relies on plant response, but incorporates chemical soil tests. We discuss each of these approaches.
Plant-Based Approaches
To determine how much of the plant’s nutrient needs can be met by the soil, plant-based approaches use measurements of K uptake. Using omission plots, the “indigenous supply” of K in the soil is found by measuring the total amount of K taken up by plants that are grown where no K has been applied but where all other nutrients are in sufficient quantities. Keeping all other nutrients sufficient ensures that other nutrients do not limit plant growth. If limitations from other nutrients did occur, plant growth and total nutrient uptake of K would be reduced and the indigenous supply of K in the soil would be underestimated.
But is the indigenous supply of K high enough? To answer this question, the indigenous soil K supply is compared to the amount of K taken up by plants receiving adequate K. If both quantities are the same, then plant-available K supplies in the soil are sufficient. If K uptake by fertilized plants exceeds the indigenous K supply, then the soil supply of K is not high enough.
The amount of K in the soil can often be quite large. Potassium is part of the atomic structure of several minerals in soils, like feldspars and micas. However, only a small portion of the total K in soils is available to plants during a cropping season. Plant uptake is perhaps the most direct measure of this plant-available supply. Because it is not feasible to put omission trials on every parcel of ground that is to be evaluated, scientists assemble data from various sites and years where such trials have been conducted and create models that help them estimate indigenous soil K supplies and total uptake requirements for areas where no data exist.
Soil Testing-Based Approaches
Soil testing is another approach to determining how much of the plant’s nutrient needs can be met by the soil. It is also built around plant response, but the emphasis has most commonly been on yield response rather than on nutrient uptake.
Soil testing was developed to provide a method of predicting whether or not K is needed before a crop is grown. The strengths of this method are its speed and its ability to be used at higher spatial resolutions. Several soil tests can be taken in the footprint of just one omission plot experiment.
Soil testing usually uses chemical solutions to remove a portion of the K from soil particle surfaces that is considered to be plant-available. This is not a direct measure of the total amount of K available for plant uptake. Instead, it is simply an index that must be related to plant response to have any agronomic meaning. Creating this relationship is accomplished with a calibration study.
In a calibration study, a representative sample of the soil is taken from the experimental site, typically to a depth of 6 to 8 inches. The soil is tested with the laboratory procedure and a result obtained.
Then one of two experiments is conducted. The first option is an omission plot, like that described above, where crop yield without K (the check) is compared to crop yield fertilized with K. The second option is a K rate study, where incremental rates of K, including a check, are applied. The first approach measures yield response only. The second approach measures not only yield response but, when combined with statistical models, the quantity of K that was needed to just reach the highest yield attainable at that site. The yield of the crop grown without K is expressed as a percentage of the yield obtained with sufficient K. This percentage, called “relative yield” indicates whether or not the indigenous supply of K is adequate. A relative yield less than 100 percent signals deficiency. The soil test level measured at that site is then associated with the observed relative yield. This association indicates what percent of the attainable yield can be met by the supply of indigenous soil K indexed by the soil test.
Soil test calibration relies upon testing a range of indigenous soil K supplies. This approach is needed to test how sensitive a chemical test is to such changes. There are two basic approaches to obtaining a range of indigenous supplies. The first is to conduct trials across a large number of sites over time. A second approach is to conduct a rate study for many years on one site. The first approach provides calibration information that can be generalized across large areas and a range of conditions and management practices. The second approach provides calibration information for a single site across many years, providing site-specific information.
Nutrient Budgets
A key component of both plant-based and soil testing-based approaches is the nutrient budget. It is calculated by subtracting the amount of K removed from a parcel of land from the quantity of K applied. Positive budgets indicate K enrichment while negative ones signal K depletion. Most often, “partial budgets” are calculated.
These simplified budgets compare 1) nutrients removed with harvested portions of plants, termed “crop removal,” and 2) K applied with commercial fertilizers, manure, and/or biosolids. These budgets are partial because they do not consider all inputs and outputs.
The degree to which partial budgets deviate from complete budgets depends on the system considered.
Potassium budgets indicate whether agricultural practices are depleting, enriching or maintaining indigenous K supplies. Where indigenous supplies of K are low, enrichment is appropriate. Depletion is appropriate where indigenous supplies are high, such as in more arid agricultural areas; however, there is a caveat to depletion. If it occurs long enough on soils with high amounts of K, the indigenous supply eventually becomes inadequate for crops.
Concern about long-term negative budgets has been expressed by others. K applications must not only provide enough K to meet crop needs, they also need to sustain plant-available soil K supplies over the long term. There are many soil testing-based approaches that explicitly factor K budgets into their algorithms
Scientific Conclusion
Potassium is required by plants. Not applying K on soils with low indigenous supplies limits yields and production and is considered a form of land degradation. On soils with high indigenous supplies, omitting K will not reduce yields or production; however, continued withdrawal of K through successive crop harvests will eventually deplete indigenous supplies to yield-limiting levels, as has been observed in several areas around the world.
Potassium fertilization is necessary. Both plant-based and soil testing-based approaches inform decisions about whether or not a K application is needed to provide plants with adequate nutrition and sustain soil productivity.