Diagnosis by deduction

Through the process of elimination, a simple diagnostic tool enables farmers to determine the kind of fertilizer their soil needs. 

There are 16 essential nutrients needed by plants. Three of the sixteen, carbon, hydrogen, and oxygen, are from air and water. The rest—nitrogen (N), phosphorus (P), potassium (K), calcium, magnesium, sulfur (S), copper (Cu), iron, manganese, zinc (Zn), boron, molybdenum, and cobalt—are from the soil. Plants will not grow normally if any one of these is not available in sufficient amount.

As yields increase, the essential micronutrients in the soil are also mined heavily as the macronutrients which could lead to their deficiency in soils. Moreover, these are not often present in most of the fertilizers in the market. As a result, soil productivity may continue to decline even with the continuous heavy application of macronutrients such as N, P, and K from inorganic fertilizers. It is therefore important to determine the nutrients that are lacking in the soil so that only these need to be applied in the right proportion.

Filling a necessity
There are several diagnostic techniques to assess the essential nutrient level status in the soil. But these are done in soil testing facilities that are limited in number and most farmers do not have access to these facilities. Furthermore, the cost of soil analysis is beyond the capacity of many small farmers and the waiting time for the results is relatively long.

Due to the limitations of the traditional diagnostic tools, I and my co-workers developed the minus-one element technique (MOET), a farmer-friendly, affordable, and quick soil analysis kit, while working as consultant at the Philippine Rice Research Institute (PhilRice). The technique was conceptualized when I was a scientist at the International Rice Research Institute based in Indonesia. There where there were no adequate laboratory facilities at the site to carry out soil chemical analysis. Only the status of  N, P, K, and S in soils could be determined.

The MOET kit includes nutrient formulations for N, P, K, S, Zn, and Cu, the elements that are known to be deficient in most lowland rice soils in the Philippines. The formulations are the minus N (that is, it contains all essential 5 elements except N), minus P, minus K, minus S, minus Zn, and minus Cu. The seventh formulation contains all the nutrient elements (“complete”).

Simplified soil chemistry
This method requires easily acquired materials such as plastic containers or pots that can hold 4 kg of wet soil samples. Soil samples are collected from the field, preferably from at least 35 random sites per hectare if the soil in the area is fairly uniform. However, more samples may be taken if the field has an apparent soil fertility gradient even if the area is less than 1 hectare. Soil fertility gradient is observed in somewhat rolling area, where the bottom portion is more fertile than the upper or sloping portion of the landscape.

The samples are taken before plowing the field so that the farmer will know what elements are deficient in the soil by the time the field is ready for planting. The soil samples are thoroughly mixed while wet and about 4 kg of the soil are placed in each container.The formulations in the MOET kit, which is available at PhilRice for about USD 4.00 each, are then mixed separately and thoroughly with the soil samples.

The farmer transplants at least five 12-day-old rice seedlings into the containers, which must be kept wet but without standing water until the plants are well established. The plants should be watered with water from the same source used to irrigate the field. After 10 days, the extra plants are removed leaving only the two best growing plants per container.

Visual evidence
Thirty days after transplanting, the farmer should be able to determine nutrient deficiencies by simply looking at the growth of the plants in the different containers and comparing them with the plants that received the complete nutrients. If the growth of the plants in all containers is uniform, then the soil is not deficient in any element. But, if the growth of the plants in, for example, the minus N container is inferior to the plants in the complete container, the soil is deficient in N.

The test should be continued for 45 days because the growth of the plants in the minus element might still catch up with the “complete” treatment. The differences among the different treatments can be further evaluated by weighing the biomass produced at 45 days for higher accuracy.

Once the farmer has determined the deficient element or elements, he or she can apply only the nutrient or nutrients based on the recommended rates normally provided by extension workers. This is not only cost-effective, but will also help minimize the adverse effects of excessive fertilizer application such as water pollution. If farmers could save one bag of fertilizer by applying only the necessary fertilizer at the right rate, they can expect an increase in yield by 1 or 2 sacks of rice, making the price of the MOET kit worth it.

Dr. Mamaril is a former agronomist (1972-1993) at the International Rice Research Institute (IRRI). After retiring from IRRI he worked at the Philippine Rice Research Institute as a senior soils and agronomy consultant for 17 years. He is now a full-time organic farmer who grows fancy rice.