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Measuring Soil Fertility and Its Interpretation

There is no denying the fact that achieving and maintaining appropriate levels of soil fertility, especially plant nutrient availability, is of paramount importance. If you want the agricultural land to stay capable of producing the best quality crops, you must make sure that appropriate levels of soil fertility and plant nutrient are maintained. Soil Index System is widely used to assess soil fertility, and there are some alternative approaches based on the ratio of certain cations in the soil. The soil cation exchange capacity (CEC) is the measure of the total negative charges within the soil that absorbs plant nutrient cations like Calcium (Ca2+), Magnesium (Mg2+), and Potassium (K+).

The Three Crucial Steps to Measure Soil Fertility and Plan Nutrients

  • Soil sampling and analysis
  • Interpretation of the analytical data
  • Recommendations for nutrient additions as fertilizers or manures to enhance crop yields and reduce any adverse impact on the environment due to their application.

The productive capacity of the soil will depend on the biological, chemical, and physical properties of the soil. A good farming practice includes managing the various factors that make up each of the three properties to optimize the production of crops in environmentally friendly ways. Soil analysis aids to manage soil nutrients efficiently, measure soil fertility, and maintaining plant nutrients like phosphorous (P), potassium (K), and magnesium (Mg) that are retained in the soil. If the amount of any of these elements is inadequate, the yield will be jeopardized.

Measuring Soil Fertility -The Factors Affecting the Chemical Properties of a Soil

For decades, scientists have sought ways to measure soil fertility and indicate the availability of plant nutrients like potassium, phosphorous, and magnesium. Soil analysis is only valuable when it is done properly according to the rules and methods of good sampling. The primary factors that affect the chemical properties of the soil are as follows:

  1. Soil acidity (pH) doesn’t affect the growth of most crops directly, but it can influence the availability of other nutrients.
  2. The total amount of soil organic matter affects the soil structure and availability of nutrients.
  3. The plant nutrients like P, K, and Mg.

As a measure of soil fertility, think of the availability of phosphorous, potassium, and magnesium in the soil as the number of pools shown in the following figure 1.

measuring soil fertility

Roots will take up both P and K from the soil, but the amount of P is very small. When P and K are depleted by root uptake, they are replenished from the reserves in the readily available pool. When this pool becomes depleted of P and K, they are replenished through the reserves in the less readily available pool. The amount of P and K in the readily available and less readily available pool will depend on the history of fertilizer and manure additions.

The critical feature in crop nutrition is that there must be enough K, P, and Mg in the soil solution and a readily available pool to meet the maximum daily demand for every nutrient in the early stages of growth and the highest uptake to achieve the optimum yield.

Measuring Soil Fertility – The Recommendations and Interpretations

When it comes to the measure of soil fertility and its interpretation, there are two ways of relating the fertilizer recommendations to soil analysis using the Index System. Both interpretations must be developed from statistically valid field experiments that relate crop response to soil data collected through reliable methods of soil analysis.

The first method

Conduct a series of experiments with the same crop on multiple sites with different levels of readily available plant nutrients P, K, and Mg. In every experiment, the yields are measured on plots with a variable amount of nutrients being tested, and all other inputs are kept the same on all the plots. An example of potassium is shown in Figure 2.

The first method of measuring soil fertility

When data from multiple experiments are assembled, there will be a wide range of K, P, and Mg levels and different responses of the crop to fresh P, Mg, and K at each soil level. The lowest soil level where there will be no response to P, K, or Mg is the critical level for the plant-available nutrients for the crop and soil type where the experiments are done.

Such experiments must be conducted on various soil types and over a wide array of growing seasons. Thus, collecting the measure of soil fertility becomes expensive. However, over time, extensive data can be collected to optimize productivity.

The second method

This is to establish the plots with a range of soil nutrient levels, i.e., different index values on one site. An example of P is shown in Figure 3. Keeping all the inputs the same, the yield is related to the soil index. The point where the curve levels off will be the critical value for the crop and soil type. Below the critical value, the yield will be lost, and it will be a financial penalty to the farmer.

The second method of measuring soil fertility

When it comes to measuring soil fertility and plant nutrient, this method defines the critical value with accuracy for that soil and farming system. This is because every experiment with its range of soil index values is within one management. Using this approach, when the soil is at the critical level or just above it, the fertilizer recommendations are based on replacing the amount of nutrient that is removed from the harvested crop.

If the soil is below the critical level, more nutrients can be added, and if the soils are well above the critical level, then nutrients can be reduced. The addition or reduction in plant nutrients will continue till the soil reaches the critical value. To ensure this is reached, the soil must be analysed and sampled every 3/5 years. In general, both approaches show that maintaining the soils at Index 2 for K, P, and Mg ensures that there will be enough nutrients for most arable crops and grassland.

In closing, measuring soil fertility and its interpretation starts with accurate soil analysis. For more information on high-resolution soil mapping, contact the experts at SoilOptix.