Regarding knowledge about the factors influencing plant growth, an understanding of soils probably is the most important one.
In subtropical and tropical area’s (this is where Jatropha belongs),variation in soil type, pH and nutrient content is huge. In the case of growing Jatropha it is even more crucial to understand (at least) the general soil conditions since there is very little research about Jatropha and soil.
Table 1. Most soils contain four basic components: mineral particles, water, air, and organic matter. Organic matter can be further sub-divided into humus, roots, and living organisms. The values given above are for an average soil in a subtropical climate.(table from PhysicalGeography.net. (5)
The first and very rough characterisation of soils is based on visual aspects. Most soils are characterised by the presence of clay, silt and sand. Depending on the mixture of these particles we characterise soils ”heavy” or “light”.
The texture of a soil refers to the size distribution of the mineral particles found in a representative sample of soil. Particles are normally grouped into three main classes: sand, silt, and clay.
Soil structure has a major influence on water and air movement, biological activity, root growth and seedling emergence.
Heavy soils (clay, silt) do have a structure determined by very small particles. When this type of soil becomes wet, it takes a long time to dry up, even when proper drainage is applied. These soils are difficult to work with, and growth is difficult to control. On the other hand, most clay soils (like black cotton) are relatively rich in nutrients. Their structure is determined by how the individual soil granules clump or bind together and aggregate, and therefore, the arrangement of soil pores between them. Soil structure, especially in heavy soils is degraded by over-fertilization, compaction trough heavy machinery and frequent traffic, and over irrigation. Never work a soil when it is to wet! In temperate area’s frost repairs a lot of soil degradation. In tropical area’s it is much more difficult to maintain a good soil structure. Organic material, which is among others important for the exchange of nutrients from the soil to the plant, is decomposed very vast by high temperatures, high humidity and intensive labour.
Jatropha curcas L. and soil structure.
Most of the literature regarding agronomy practices for Jatropha curcas states that the plant grows best on well drained sandy soils. This however is nothing special, since at least 50% of all the plants all over the world prefer the same growing conditions. On top of that, cultivation in light sandy soils makes it more convenient for men. Water and nutrient levels can be maintained quit easily. In spite of the above, Jatropha grows well on clay soils, as long as there is proper drainage and/or run off.
Light soils like sand are based on a very course particles size. This means that water drains very fast which is a good thing if you have enough water. On the other hand, a lot of water and good drainage leaches the available nutrients very fast, so light soils mostly are poor soils.
Soils support a number of inorganic and organic chemical reactions. Many of these reactions are dependent on particular soil chemical properties. One of the most important chemical properties influencing reactions in a soil is pH. Soil pH is primarily controlled by the concentration of free hydrogen ions in the soil matrix. Soils with a relatively large concentration of hydrogen ions tend to be acidic. Alkaline soils have a relatively low concentration of hydrogen ions. Hydrogen ions are made available to the soil matrix by the dissociation of water, by the activity of plant roots, and by many chemical weathering reactions.
Table 2.The pH scale. A value of 6.5-7.0 is considered neutral. Values higher than 7.0 are increasingly alkaline or basic. Values lower than 6.0 are increasingly acidic. The illustration above also describes the pH of some common substances. (Source: PhysicalGeography.net (5)
In subtropical and tropical area’s variation in soil type, pH and nutrient content is huge. Since a lot of Jatropha is not planted on agriculture land, you may end up with unused or degraded land. fertility and soil structure could become a concern. Below you will find a couple of soil samples in various tropical countries with some common characteristics. (all samples taken by the author and analyzed by BLGG in Oosterbeek, Netherlands)
All the samples have one characteristic in common : There is a structural lack of Phosphorus. The role of Phosphorus in perennial crops is highly underestimated. In the ornamental horticulture it is very well known that a sufficient supply of Phosphorus guarantees a better root system, more branching and more flowering. In annual oil seeds like sunflower sufficient P increases both yield and oil content. (See literature: Phosphorus requirements for sustainable agriculture in Asia and Oceania (15 MB))