Coffee is cultivated in different Agro climatic regions receiving moderate to heavy rainfall. The soils where coffee is cultivated ranges from sandy loam to clay. Most soil analysis reports give a positive status on the availability of K in coffee soils.
The paradox faced by the Coffee Farmers is such that the Potassium content of soil is usually many times greater than the amount taken by the coffee plant and other multiple crops associated with coffee during the growing season. In most cases only a small fraction of it is available to plants. This article highlights the important pathway of potassium dynamics in Coffee soils and the prerequisites needed to make it available for growth and development of coffee and allied crops.
Plant roots take up nutrients from the soil solution. The absorption of potassium from the soil solution to the plant root system is by diffusion and mass flow of potassium to plant roots. The amount of potassium that can diffuse is directly related to the intensity of potassium in the soil solution. Diffusion in many soils accounts for 88 to 96 % of Potassium absorption by roots.
Potassium diffusion to roots is limited to very short distances of a few millimetres from the root surface in soil. The presence of Potassium further away in the soil system, although possibly plant available, is not positional available.
Only a small fraction of the potassium requirements of coffee is available by direct contact of the roots with the soil medium. In fact the bulk of potassium needed by the coffee plants has to be transported in the soil to the roots in the presence of adequate moisture. Coffee farmers need to understand that the water holding capacity of soils should have optimum levels of moisture for the transport and availability of potassium ions into the coffee plant. Since all coffee plantations in India are shade grown with multi crops and different layers of canopy shade, the floor of the coffee forest is a dense mat of roots. Robusta plants have a denser mat of roots with a large number of surface feeder roots in comparison to Arabica. Hence, application of split dozes of potassium fertilizers during sprinkling or backing showers, ensures efficient uptake of fertilizer potassium because of the optimum levels of moisture available in the soil system.
The actual mechanism of potassium transport occurs mainly in the soil solution by mass flow and diffusion along a concentration gradient that is built by the absorbing root. In the immediate vicinity of the roots, the soil nutrients are rapidly taken up by the plant, due to absorption, there by depleting the nutrient pool. Continuous potassium supply to the growing plant is only ensured when the rate of potassium release to the soil solution and transport to the roots keeps pace with the rate of nutrient uptake.
Other factors that influence potassium diffusion and thus potassium availability to coffee and mixed crops includes soil properties, type of soil, organic matter content, temperature and host of other factors including environmental conditions.
The type of soil where coffee is cultivated makes a significant difference in the availability of potassium. The more potassium ions are dissolved in the soil solution, the more are available to the plants. The K concentration of the solution depends on the release of potassium from soil minerals. Clay minerals are the most important source of soil K. They hold the bulk of mobile K and release it when the concentration of the soil solution falls due to plant uptake or to an increase in soil moisture. Soils having optimum soil moisture regimes aid in efficient uptake of the cation. A good K saturation of the clay minerals results in an equilibrium with a high K concentration of the soil solution, where as poor K saturation is in equilibrium with low K concentration in the solution.
The capacity of the coffee soils to make available required amounts of potassium into the soil solution depends on the nutrient saturation of the clay particles. As a rule of thumb, clay minerals well supplied with potassium ensure an adequate potassium concentration in the soil solution. According to the research data published by the International Potash Institute, for equal saturation of the clay minerals, heavier soils need much more potassium than light soils because of their higher clay content. Their study reveals that the relationship between exchangeable potassium and potassium concentration in the soil solution of two soils clearly shows that a heavier soil requires far more exchangeable potassium than a sandy soil to reach the same potassium concentration in the soil.
Review of literature on potassium dynamics, especially from the International Potash Institute has clearly elucidated the role of potassium under field conditions and how the dynamics is affected by a host of factors. Soil scientists are of the view that the composition of the soil solution can change rapidly due to variations in climate, soil moisture, and nutrient uptake by plants, temperature and other factors.
Coffee Farmers need to be aware of the fact that the exchangeable or available potassium as determined by soil analysis, does not give satisfactory information on the level of the actually available soil potassium unless related to the clay content and to the nature of clay minerals. Heavy soils require considerably more fertilizer potassium to attain a high level of potassium availability than light soils. On the other hand, clay soils possess a better potassium buffering capacity than sandy soils.
Research Experiments at the International Potash Institute (IPI), have many relevant findings for the Coffee farmers worldwide. We report the findings of the IPI.
With the same quantity of exchangeable potassium present in the soil, the concentration in the soil solution may be different, depending on the capacity to adsorb potassium. At equal levels of exchangeable potassium, a light soil will release potassium at a higher rate into the soil solution than a heavy soil. The same is true for added fertilizer potassium.
For crop production it is of importance whether the concentration of the soil solution decreases rapidly during the growing season or is maintained at a fair level=well buffered. Sandy soils show poor potassium buffering and may need repeated potassium dressings or a higher initial potassium concentration in order to ensure an adequate concentration during later growing stages.
When the exchangeable potassium is depleted in the root zone, plants can take up potassium which was initially non exchangeable. But the speed of release from this source into the soil solution is too low as to sustain a high yielding crop.
Soils low in potassium or in areas where potassium application has been restricted for many years, subsequent potash applications will not make it available to plants. On the contrary heavy soils do not only strongly bind the fertilizer potassium but even fix it. The lack of yield response to usual potash dressings sometimes observed on soils testing low or medium in available potassium van often be ascribed to the strong fixation of fertilizer potash in the soil. This applies particularly to clay soils. To overcome this problem, the only feasible solution is the applications of extremely high doses of potassium fertilizers.
Diffusion is one of the major factors of potassium movement in the soil. It increases with improved potassium saturation of the clay minerals. Diffusion proceeds faster in a light soil than in a heavier soil. The diffusion rate also depends on the moisture status of the soil. In a relatively dry soil more potassium has to be given in order to overcome the decrease in potassium mobility. This is of special importance in rainless periods of short duration. Due to impaired movement of nutrients, potash deficiency and corresponding losses in yield and quality will occur in case of insufficient potassium saturation. In a wet soil additional potash supplies help to counterbalance the diminished nutrient uptake capacity of the roots caused by poor aeration and to avoid unfavourable reduction processes in the soil.
Restricted nutrient availability due to unfavorable soil moisture conditions can be counterbalanced to a certain extent by improving the potassium status of the soil by fertilizer application. Improving potassium availability by potash application is essential, particularly in case of unfavorable soil moisture.
Coffee Forest Root System Ecology
Indian shade grown eco-friendly coffee is a mix of multiple crops closely associated with a population of heterogeneous forest trees both introduced and native. Also, the per acre or hectare plant population of Arabica or Robusta is pretty high leaving no space empty in a given unit area. In situations where the plant population is spaced apart, then the gaps are used to cultivate multiple story crops like Areca, Cardamom, Pepper, Vanilla, nutmeg and many other fruit crops. It is of paramount importance to understand that the floor of the coffee forest is not only covered with leaf litter but within a few inches from the top soil, a dense mat of coffee roots, runs for hundreds of kilometres, intertwined with mixed crops and forest trees . If one were to dig a few centimetres of soil, it is easy to see the thick mat of roots spreading all across the coffee forests. Research indicates that the coffee plants roots can reach up to 20 to 25 kilometres in length with an impressive absorption range of 500 meter squared.
The Coffee plant is genetically gifted with three types of roots, namely Vertical root, lateral root and tap roots. Each type of root system has a definite role to play in the absorption of nutrients from the soil. The tap roots generally burrow the top layers of soil and can be seen within 40 to 50 centimeters from the surface. Four to eight axial roots may be present which grow downwards. The lateral roots grow about 2 meters from the plants trunk. Feeder roots can be observed in the subsurface region of the soil, usually 20 to 25 centimeters from the top soil and can be found about 100 centimeters away from the main coffee plant.
Also, we are of the opinion that due to the complexity of the type of trees, herbs, shrubs and mixed crops associated with coffee, there is a constant mining as well as recycling of nutrients from the deeper layers of the soil to the top soil there by facilitating a strong build-up of a nutrient pool, which influences coffee productivity.
One common practice observed in both Robusta and Arabica coffee plantations is the creation of cradle pits or water imbibing trenches to retain water during the dry season. By doing so, the coffee farmers help enhance the growth of horizontal roots due to the clipping of vertical roots during the cultural operations. This practice is very important and needs to be carried out once in 5 years because it gives new vigour to the plant system and allows the plant to harvest water and minerals from the soil nutrient pool. This practice also helps the dense growth of horizontal roots that remain concentrated in the surface to subsurface region of the soil which plays a very crucial role in the growth, development and yield potential of the coffee plant.
The fact of the matter is all shade grown eco-friendly Indian Coffee is heavily dependent on monsoons to meet their water requirements. It is also a fact that in recent years, due to the impact of global warming, the possibility of obtaining an ideal soil moisture status for the uptake of nutrients, especially potassium uptake may not exist under field conditions. Two scenarios may exist. First, due to insufficient rainfall, to overcome low mobility of potassium in the soil, extra split applications of fertilizer potash needs to be applied. Second due to heavy rainfall or cloud bursts leading to an excess moisture regime in the soil system which in turn leads to restricted absorption power of roots caused by oxygen deficiency, external application of two to three rounds of potassium fertilizer is a must.
Anand T Pereira and Geeta N Pereira. 2009. Shade Grown Ecofriendly Indian Coffee. Volume-1.
Bopanna, P.T. 2011.The Romance of Indian Coffee. Prism Books ltd.
Dr. G. Kemmler and H.Hobot, Ing.agr.,Buntehof Agricultural Research Station, Hannover, Fed. Rep. of Germany. Potassium in Plant Production.
Dr. G. Kemmler and H.Hobot, Ing.agr.,Buntehof Agricultural Research Station, Hannover, Fed. Rep. of Germany. Potassium dynamics in the soil.
Malavolta, E. Nutrição Mineral, Calagem, Gessagem e Adubação do Careeiro. Summary in English.
Nutman, F. J. 1933. The Root System of Coffea Arabica L. I: Root systems in typical soils of British East Africa. II: The effect of some soil conditions in modifying the normal root system. Emp J Exp Agric. 1: 271-84, 285-96.