Maria Violet D'Souza and Jayarama
Cultivation of coffee on the same soil, over a long period, without adequate fertilization will reduce yield and quality of the crop considerably. It is a well known fact that soil fertility varies from soil to soil, block to block and region to region. Higher yields and quality coffee can be expected only in soils of higher fertility and where intensive fertilizer programme is followed with adequate soil replenishment activities.
Coffee soils
For coffee, soil depth should be at least 1-2 meters to allow the tap root development and ensure the necessary water supply to the plants during the dry spell of the year. Soils where the water table however, is permanently high should not be used for planting coffee. The water table shall be at least 1.5 - 2.0 meters below the soil surface. Soils which are constantly subject to flooding should be avoided as coffee tree roots are easily suffocated. Badly drained soils or heavy soils with excessive clay content are not good for coffee cultivation. In areas with very low average annual rainfall or a long dry season, sandy, rocky or heavily laterized soils with poor water retention are also not suitable for coffee. Since the main part of the radicular system of the coffee tree develops in 30-45 cm of the upper soil layer, the physical, chemical and
biological properties of the top soil are more crucial to the coffee plants than those of the deeper subsoil. Soil porosity is also important and soils with porosity 50-60% (water + air), 45% mineral content and 2-5% organic content are best suited for coffee plant growth. Soils with high percentage of organic materials are best suited for coffee as they are more fertile, assist the assimilation of applied fertilizers, less prone to erosion and offers better water and nutrient retention. In summary, ideal soils for coffee should be deep, permeable, slightly acidic and porous. Once the soils are planted with coffee, the soil properties have to be monitored on constant basis as coffee plants remove lot of minerals and other nutrients from the soil and these nutrients have to be replaced with adequate external application to maintain soil fertility balance. This can be done by routine soil analysis.
Soil testing
Soil analysis, plant tissue analysis, pot culture and field experiments are the four different approaches adopted to evaluate the soil fertility status.
Soil testing has long been recognized as an important tool to assess the available nutrient status. It involves rapid chemical analysis, interpretation of results, evaluation and fertilizer recommendation. The primary function of soil testing is to make soil test based Soil pH and rating
The soils with less than pH 7.0 are called acidic, while normal soils have the pH ranging between 6.0 and 7.0. The soils tend to become alkaline when pH goes beyond 7.0. For coffee the optimum soil pH should be around 6.1.
Causes of soil acidity in coffee plantations and its effects
The acidic parent material from which soils are derived may be responsible for soil acidity. The loss of calcium and magnesium by leaching, erosion or by crop removal result into acidic soil. Continuous use of acid forming fertilizers and degradation of plant residues also can cause reduction in soil pH.
Once the soil is rendered acidic for any one of the reasons, the availability of essential nutrients required for the growth of the coffee plant is restricted and this affects the performance of the plant in the field. For example some of the repercussions of unattended soil acidity are:
1. A decrease in nitrification rate and reduced availability of nitrogen to plants
2. Increased phosphorous fixation and retention in soils
3. Reduced potassium activity
4. Low availability of magnesium which is essential for chlorophyll generation.
5. Release of elements like aluminum and manganese in toxic levels.
6. Low soil microbial activity
7. Appearance of nutrient deficiency symptoms in plant tissues
8. Soils becoming marginal and unsuitable for coffee plant growth
9. No response to the applied inputs
10. Decreased crop yield with higher incidence of pest and diseases
Correcting soil acidity
Once the soil acidity goes beyond the desired level, the soil tends to become unproductive or lose its nutrient supplying capacity. So, optimum soil acidity has to be maintained, which is favorable for the microbial activity in soils. Correction of soil pH improves the physical properties like structure and permeability of soil which in turn help in the easy flow of nutrients.
The only method widely accepted or practiced in combating soil acidity problems is liming. Liming in simplest terms is nothing but addition of calcium and magnesium containing compounds that are capable of counteracting the ill effects of excess hydrogen ions (which cause acidity) present in the acidic soils. In contrast when pH goes beyond 6.2, then acid forming fertilizers can be applied as nutrient sources to correct soil pH. Application of Gypsum (calcium sulphate) or Epsom (magnesium sulphate) in required quantity also will help in lowering the pH values.
Several direct and indirect methods are available to indicate the lime requirement of coffee soils. Laboratory studies at CCRI indicated that Shoe-Maker's method is the best suited method for determining the lime requirement of soils cropped to coffee. Soils with pH less than 4.2, a full lime requirement rate is needed to bring the pH level to 6.2, while half and 1/31d lime requirement rates were found enough for soils of pH 5 and 5.6 respectively to bring the pH level to 6.2. Any soil with a pH value below 6.2 (in soil: distilled water suspension) need to be tested by Shoe-Maker's method for lime requirement and soil with pH above 6.2 does not need liming. One should remember that lime reaction in soil is a slow process and largely depends on the soil texture, property, soil temperature and moisture.
Commonly used liming sources in coffee
Choice of liming materials primarily depends on the cost per unit of neutralizing value. Neutralizing value refers to the relative capacity of particular liming material to neutralize a certain amount of acid forming hydrogen ions in the soil. The degree of fineness of the liming material is also important as finer the particles, quicker will be the reaction in soil. A liming material of which 80% passes through a 60 mesh sieve or at least 90% through a 20 mesh sieve is suitable for liming the soils.
Different liming materials are available in the market, namely, calcite lime, dolomitic lime, burnt lime, hydrated lime and basic slag. Their relative neutralizing values in the above order are 80-100, 80- 125, 150175, 120-135 and 50-70 respectively. Dolomitic lime stones are suitable for soils with low Magnesium content. However, the reactivity of liming materials depends on its particle size, method of application, neutralizing value and moisture content in the soil. Continuous application of dolomitic lime is not recommended as it interferes in the up take of potassium. Every year application of liming material is not suggestive.
Frequency of liming in coffee fields
Regular soil testing once in 2-3 years helps the coffee farmers to decide the liming needs. Liming however, is recommended at least once in 2-3 years in coffee plantations after the first application. Without soil test data, liming will be a futile exercise. Liming of coffee soils may be done any time during the season (except during rainy season) provided sufficient moisture is present in the soil. Generally liming application is more effective during November month in Indian coffee plantations where sufficient moisture will be present in the soil. Agriculture lime which analyses a minimum of 80% calcium carbonate equivalent is suitable for liming. The soil testing laboratories of the Coffee Board have the facilities for analyzing these chemicals on nominal rates for the planters.
Method of application
Liming materials may be evenly broadcast in the field and incorporation to the soil with light forking for quicker and better results. While procuring liming materials from the industrial sources, care should be taken to find out the impurity levels in the materials. Liming materials should not contain more than 0.7% sodium as if affects the soil structure adversely. Application of lime in 2 or 3 splits can be practiced when large quantities of liming materials are needed (more than 2.5 M.T) to correct soil acidity. A maximum of I M.T per annum per unit area can be applied in one split.
Advantages of liming coffee soils
- Liming provides calcium for coffee plant growth and improves the root growth.
- It increase nutrient availability and efficiency of applied fertilizers
- It increases the microbial activity and soil productivity
- It reduces toxicity of elements like aluminum and manganese in soils
- It improves the physical condition of the soil
However, excess liming has its own demerit or adverse effect on the soil and plant system. If the soil pH is greatly increased by the addition of excessive amounts of lime, phosphate availability will be decreased considerably due to precipitation of phosphate as calcium phosphate which is highly insoluble.
Over liming also results in deficiency of available iron, manganese, copper and zinc. Excess lime may cause disturbance in uptake and utilization of boron due to antagonism between calcium and boron. Blanket application of lime without soil testing is not advocated.
The most scientific, timely and methodical liming of the coffee fields has always the advantage of better crop production and adds to the economy of the cultivation. Growers should recognize lime as one of the most important part of the production inputs in the coffee farming system, considering the benefits of liming. Thus, timely liming of the field can serve as a powerful and cost effective tool for better fertilizer use efficiency in coffee plantations and maintaining the soil health for a very long time.
