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The potential of potash

With high clay content, vertisols require much higher K levels to meet plant needs (IPI)
With high clay content, vertisols require much higher K levels to meet plant needs

Potash, originally derived from the Dutch word potasch for wood ash, was the term given to the pioneering practice of extracting potassium fertiliser (potassium carbonate K2CO) by leaching wood ashes and evaporating the solution in large iron pots. Today, the majority of the world's potash is supplied from deep mines extracting sedimentary rock deposits. However, whilst potassium (K) is a vital nutrient for efficient plant growth and crop production, K management is often poorly understood and fertiliser recommendations can be rather generalised.

Across the black soils (vertisols) of central and southern India, rainfed agriculture supports the production of a variety of crops, including sorghum, soyabean, cotton, maize, groundnut, chickpea, sugarcane and citrus, amongst others. Vertisols, with high clay content, require much higher K levels to meet plant needs, compared to lighter texture soils, such as those that occur in the northern part of India.

Outdated recommendations are needed for a change

Traditionally, K recommendations in India have been based on soil tests conducted predominantly in the alluvial Indo-Gangetic plains. The critical levels to classify soils into low, medium and high category of K were established over 50 years ago and were recommended for all the soil testing laboratories throughout the country irrespective of the soil type, crops grown and environmental conditions. Soils testing for less than 120 kg K ha-1 were considered as low-K soils, testing 120-280 kg K ha-1 as medium-K soils and more than 280 kg K ha-1 as high-K soils.

Examining soil profile is a vital tool for assessing soil properties (A. Naumov/IPI)
Examining soil profile is a vital tool for assessing soil properties
A. Naumov/IPI

In general, it was expected that soils testing low in available K should respond to the application of K to a greater extent, soils in medium category respond to K application to a lesser extent, and soils in high category may not respond to K application. The K availability map for India shows greater number of soils in high category in vertisols. Since these soils were classified as high K soils no K application has been recommended for them. However, long-term experiments conducted on groundnut, wheat, sorghum, chickpea, rice, cotton, sugarcane and soybean in vertisols have showed a good response to the application of K.

The majority of vertisols in India are found in Maharashtra, Madhya Pradesh, Andhra Pradesh, Karnataka, Tamil Nadu, Gujarat, Rajasthan and Chhattisgarh. With no recommendations being made for K across these states, the sustainability of crop production was threatened in the absence of K application, especially under irrigated conditions.

A greater need for K

The reason for the variation in K response in alluvial and vertisol soils can be explained by the different nature of the soils. To a layman, soil may look like soil but the physical and chemical characteristics actually vary quite considerably. Alluvial soil has a high content of large sand particles, whilst vertisols consist of smaller, finer clay particles. The finer the particles, the more K is required before it becomes available to the plants. Rather like a piece of silk cloth, compared to a sponge cloth - more water is taken up by the sponge before it is released.

Vertisols rich in clay minerals, like a sponge, require more K before it is made available to the crops it can support. For example, a low application of K (15 mg K) will readily saturate sandy soil, but leaves many clay particles unsaturated in the clay soil, leaving no available K for plant uptake.

Refining the recommendations for potash

Within this context, the International Plant Nutrition Institute (IPNI), International Potash Institute (IPI), and the National Bureau of Soil Survey and Land Use Planning (NBSS & LUP) of the Indian Council of Agriculture Research (ICAR) organised a workshop in Nagpur from 24-25 August, 2010 to assess the requirement for changing K fertiliser recommendations in vertisols in India.

K is a vital nutrient for efficient plant growth and crop production (IPI)
K is a vital nutrient for efficient plant growth and crop production

Around 40 scientists from universities in the vertisol regions, ICAR and state officials were invited to discuss this issue with the aim to re-align fertiliser recommendations to maximise farmers' income and maintain soil fertility. Papers were presented including coverage of the special mineralogy of vertisols, K requirements of crops grown, typical responses to K applied, and suitability of the recommendations with regard to the actual responses reported over many years.

After frank discussions amongst the participants, it was unanimously agreed that the present threshold for K fertiliser recommendations in vertisols (55 ppm as the limit below which K is given) needs to be significantly increased, even doubled to 120 ppm (approx. 250 kg ha-1). It was further proposed that more research is required to refine this recommendation. Work conducted by IPI, IPNI and Indian scientists from universities located in regions with vertisol soils will help to substantiate this recommendation.

"Potassium fertiliser recommendations must bring into account site-specific plant response," says Hillel Magen, IPI director. "These responses vary with soil type, precipitation and climatic conditions, crops and management. I strongly believe that looking critically at soil type (like vertisols) and modifying K recommendations accordingly, will bring a substantial improvement in agricultural production."

With contributions from: Dr. M.S. Brar, Punjab Agricultural University, India and senior consultant to IPI

Date published: November 2010


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Apart from age old unrelated recommendations, the attitude o... (posted by: Kskarnic)


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