Soil testing made accessible for farmers

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Soil testing allows farmers to identify the nutrient content of their soil, thereby helping them make better decisions with what amendments to apply. It may seem like a daunting task, but there are both low-tech and high-tech solutions that farmers can use so they can test their soil on their own.

Techniques how soil testing is conducted are discussed in the book “The Philippines Recommends for Soil Diagnosis and Recommendation” published by the Department of Science and Technology (DOST) – Philippine Council for Agriculture, Aquatic and Natural Resources Research and Development (PCAARRD). The book goes through the minus one element technique and chemical analysis.

Minus one element technique

The minus one element technique (MOET) is based on the law of the minimum developed by agriculture scientist  Carl Sprengel. The law states that plant growth is not dictated by the overall resource available for the plant, but by the scarcest resource, or the so-called “limiting factor.”

By applying MOET, farmers determine what are the “limiting nutrients” or the nutrients that their soil is deficient in. MOET, however, does not indicate the amount of fertilizer to apply but only helps farmers identify what nutrients their soil is deficient in. 

There are readily purchasable MOET kits that contain seven chemical formulations. The first formulation has all the essential nutrients, mainly nitrogen, phosphorus, potassium, sulfur, zinc, and copper. The other six formulations contain the same nutrients but each lack one certain nutrient so that one lacks nitrogen, the next lacks phosphorus, and so on and so forth.

Farmers will have to grow plants using the seven chemical formulations. The plants that underperform compared to the plants fed with the complete nutrients reveal which nutrients the soil is naturally deficient in. As an example, if a plant was fertilized with a “minus nitrogen” fertilizer and underperformed, this means that the soil is naturally deficient in nitrogen.

Chemical analysis

A more advanced and less time-consuming method of testing the soil is through chemical analysis. This can be performed in a soil testing laboratory or through a soil test kit (STK). When done through a lab, specialized equipment is used to extract the nutrients from the soil in order to provide a quantitative soil analysis. Soil is analyzed through a variety of parameters including the pH level, electrical conductivity, macronutrient content, micronutrient content, exchange acidity, and lime content. 

On the other hand, testing with an STK is a more accessible method that farmers can do by themselves. Farmers can determine the level of nutrients in their soil through chemical reagents that change color upon reaction with their soil sample. STKs, however, do not reveal numerical values on the nutrient content of their soil. The kit can only guide farmers with qualitative descriptors to indicate if the soil has low, medium, or high levels of certain nutrients.

Soil sampling done right

For both MOET and chemical analysis to yield accurate results, soil samples should be collected in a systematic approach so as to accurately represent the soil of the entire farmland. This would not be so complicated if farmlands did not have varying levels of fertility. For sampling purposes, the land should be divided into partitions where the soil has uniform characteristics. This means that portions of the land with the same cropping history, fertilizer treatment, soil texture, and degree of erosion should be grouped together. In rolling landscapes, the land should also be divided into the top, sloping, and bottom sections.

Soil samples should be collected from each partition of the land. For MOET, PCAARRD recommends that farmers collect 35kg of soil for each portion. They need this much because they will have to grow plants using the collected soil. For chemical analysis, farmers are advised to dig 10 random pits in each portion, then mix them up before sending to a lab or testing with a kit. The samples should be collected at least 20cm deep as nutrients are concentrated in the topsoil. Farmers should also dig for samples away from areas with organic material such as animal manure or straw.

Soil testing alone is not enough

Soil testing alone is not enough to fully familiarize oneself with the soil of their farmland. There are other factors in the soil that affect plant growth. Farmers should also consider the characteristics of their soil such as its texture, structure, color, and drainage capacity, as well as microbiological parameters like the amount of microbial biomass and level of microbial activity.

There are also other means of evaluating the soil through visual assessment. Rice farmers, for example, can use the leaf color chart to determine if they should apply more nitrogen fertilizer. If the leaves are dark green, this means the soil has enough nitrogen. Farmers would only have to adjust the leaf color chart for rice variants with inherently yellowish leaves. 

Soil may be complicated, but understanding its chemical and biophysical properties will help guide farmers making the right decisions and help them diagnose growth issues later on. 

The information provided here was taken from “The Philippines Recommends for Soil Diagnosis and Recommendation” published by the Department of Science and Technology (DOST) – Philippine Council for Agriculture, Aquatic and Natural Resources Research and Development (PCAARRD).

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