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close this bookSoil Conservation Techniques for Hillside Farms (Peace Corps, 1986, 96 p.)
close this folderSoil fertility and its maintenance
View the documentIntroduction to soil fertility
View the documentChemical fertilizers
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Introduction to soil fertility

Soil fertility is a concept based on the amounts of essential nutrients available to the plants. The plant nutrients involved are classified as macronutrients such as nitrogen (N.), phosphorous (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S); or as micronutrients such as i iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), boron (B), and molybdenum (Mb). The amount of each of these nutrients is not the only factor in determining whether there are nutrient shortages. Soil pH (acidity) is also important because under certain pH conditions, nutrients form insoluble compounds which are unavailable to be taken up by plant roots. A shortage of any of these elements in the soil can slow crop development and reduce yields, something which extentionists should consider when working with farmers to improve crop production.

In practice, soil fertility is determined by collecting a soil sample and submitting it for chemical analysis. Since a small sample will be used to infer the characteristics of an entire field, care should be taken to ensure that the sample represents as accurately as possible the conditions present in the plot.

Sampling may be done with a soil probe or a shovel, being careful to exclude the surface litter layer and sampling at a depth of approximately 1"-8" below the surface. Subsamples should be taken from random positions throughout the entire field (10-20 per hectare) and mixed thoroughly in a bucket or bag. From this mix, a 1-2 lb. sample is air dried, labelled, and submitted to a laboratory for analysis. A highly variable field should be subdivided into fairly uniform sections which should each be treated separately, as they may have very different soil characteristics. Samples may be analyzed with portable field test kits, if available, but it is also advisable to familiarize farmers with the laboratory facilities available to them. In Honduras, for example, soil analyses are performed at the laboratories of Recursos Naturales in Tegucigalpa, Standard Fruit Company in La Ceiba, the Escuela de Agricultura Panamericana in El Zamorano Valley and FHIA in La Lima, Cortes. The most common soil analyses performed are those for nitrogen, phosphorous, potassium and pH, which generally are most important in limiting crop yields. If crops do not appear to respond to changes in these factors then perhaps a deficiency of another nutrient is involved and a mere detailed analysis can be carried out.

Chemical fertilizers

A simple way to correct some soil nutrient deficiencies is through the application of chemical fertilizers. Their expense may discourage or limit their use in many areas, but applications in small experimental plots are useful in demonstrating their potential and evaluating their feasibility in a specific area.

The meet commonly encountered fertilizer are in granular form and contain compounds of nitrogen, phosphorous, potassium, or a mixture of the above. The type of fertilizer and the amount to be used generally are given as recommendations accompanying soil analysis results. In applying such fertilizers, it is important to calculate from the given recommendation the quantity that must be applied in each furrow or at each plant to avoid under- or overapplications.

The timing of fertilizers applications is also important. For example in corn, a formula containing both phosphorous and nitrogen is usually recommended at planting time, while nitrogen only, in the form of urea, is usually recommended about one month after planting. Since fertilizer recommendations vary widely based on the soils and the crop to be planted, specifics concerning their use are not discussed here. A mare thorough discussion of fertilizer use is given in "Soils, Crops, and Fertilizer Use: A Guide for Peace Corps Volunteers".

Extensionists should familiarize themselves with the fertilization practices in use throughout their zone in order to determine if changes in the types, amounts, and/or timing of fertilizer applications would be beneficial.


The use of organic fertilizers is another method of correcting soil nutrient deficiencies. Although it is more labor intensive than chemical fertilization, it has the advantages of being locally produced; maintaining soil humidity; improving soil organic matter content, structure and texture; adding a better balance of micronutrients, and improving the soils capacity to hold nutrients. Organic fertilizer refers to a number of types of organic matter which can be incorporated into soils or left as a mulch on the surface. This may be as simple as incorporating crop residues or manure, or as complex as planting a green manure crop, making compost, or raising earthworms to produce materials which will be incorporated into the soil at some later date.

A traditional practice in many areas is the burning of residues left when initially clearing land or after a harvest. Although some nutrients are returned to the soil in the ashes, much of the organic matter and nitrogen present are lost to the air. m is loss of organic matter affects all of the just mentioned soil properties.

Manures and crop residues

The incorporation of crop residues or animal manure is a means of utilizing often wasted by products of the agricultural process. They are relatively simple operations, requiring only the labor to collect and spread manure or crop residues before carrying out the normal tillage operations.

Green manure crops

Green manure crops are nitrogen-fixing plants which can be grown for a period of time and then turned into the soil as a source of nitrogen and organic matter. If cultivated for seed production and only turned into soil after dried, they contain little nitrogen and provide mostly organic matter. Table 7 lists a variety of possible green manure crops, all different types of legumes.

In crowded areas, where there is pressure to produce as much as possible on every cleared land space, it is often difficult to convince farmers to take land out of food production in order to plant a green manure crop. Fortunately, there are alternatives which allow for the production of green manures without affecting the normal crop production cycle. Some species, such as Gandul, Rice Bean, and others (especially if planted sparsely) can be grown in intercropping systems with other crops (Fig. 35). In these cases, the green manure crop can be useful as a ground cover or living mulch which reduces rainfall impact and thereby soil erosion; in reducing weed growth; and in providing useful organic matter, seeds, or forage.

Fig. 35. Green manure used as intercropped living mulch.

Some species, such as Kudzu and Velvet Bean are such aggressively climbing, densely foliaged plants, that they are best grown alone. These can be planted during fallow years and then incorporated into the soil Velvet Bean and other drought tolerant specie can even be planted near the end of the growing seas allowed to grow during the dry season, finally to be incorporated into the soil prior to the next planting season

Fig 37. Green manure tree species used in agroforestry system.

Fig. 36. Green manure used as fallow season ground cover.

Tree species, such as leucaena, madre de cacao and gualiqueme, can be planted along the edges or in contour strips in the middle of fields, providing a source of leaf material for green manure or forage, and also firewood. (Fig. 37)


TABLE 7 ( continued)


Composting is a very labor intensive technique which involves mixing plant residues, manure, soil, and water.

Decomposition takes place over a period of 1-4 months, reducing the mix to rich humus to be incorporated into the soil. Because of the bulkiness of the material and the labor involved, it is most practical in small, intensively worked vegetable plots, but it can also be used as a fertilizer in large fields of any crop if labor is available.

A compost pile can be composed of almost any organic materials: weeds, banana stalks, sawdust, coffee pulp, corn or bean residues, etc. Manure from cows, horses, chickens or rabbits, is usually used as a nitrogen source. Ashes, eggshells, and sometimes chemical fertilizers may also be added. Composting can be done with a careful eye on the chemical composition of the ingredients in order to provide an optimum ratio of the elements necessary for the bacterial and fungal decomposers, or by just throwing a little of everything in. Some farmers feel more comfortable with the casual, less exact approach, while others may be mare intrigued by a more scientific attitude towards the process.

Some of the types of compost piles are the square aboveground compost pile, the underground compost pile, the volcano compost pile, and the contour compost ditch. (Fig. 38)

Fig. 38. Various types of compost piles

The square aboveground compost pile is tightly compacted into a wooden frame and is often composed of three 12" layers. Each of the layers is made up of 9" of plant materials (both fresh and dry), 2" of manure, and 1" of black soil. Each layer is thoroughly compacted and saturated with water during construction. Posts are placed vertically in the pile (one per m ) to be removed after construction to serve as air vents. These piles are generally turned once a month and are ready to use in 3-4 months.

The underground compost pile is similar, but is constructed inside a hole dug in the ground (50-100 cm deep). This can be very effective in arid zones or during the dry season, as it reduces the amount of evaporation from the compost.

Another type of compost pile is the volcano compost pile, which is simply a mound composed of the same materials (plant matter, manure, soil and water). This type of compost pile can be used to decompose materials much more rapidly (1-2 months), if it is turned and mixed frequently (every 2-3 days). m e contour ditch compost method simply involves filling contour ditches with crop residues, manure, and soil. This method can be very effective an a large scale, leaving the materials undisturbed to gradually decompose over the course of the year, Drainage ditches, should not be filled with compost during rainy periods. Contour retention ditches can be filled however, as the piled up compost serves as a spongy retaining material. The water collected also speeds up the decomposition process.

Composting with earthworms

Another method of improving soil chemical and physical properties is the raising of earthworms and the subsequent incorporation of both the worms and the rich soil they are raised in. Earthworms can be raised in a mixture of the same materials used in composting, but in this case they are not compacted. They are also often enclosed in a box or tub to prevent their escaping. If a variety such as "Red Wiggler" (Coqueta Roja) is available, it may be much more active and reproduce much faster than other native types of earthworms. m e earthworms ingest organic matter, digest it, and leave behind a mineral rich feces which provides nutrients for plant growth. Also by physically passing through the soil, earthworms increase soil aeration and drainage.

For a more thorough discussion of composting and of composting with earthworms, see The Encyclopedia of Organic Gardening (Rodale, 1971).