Soil Fertility Is the Key

Improvement in agricultural sustainability requires, alongside effective water and crop management, the optimal use and management of soil fertility and soil physical properties (Figure 7-3). Both rely on soil biological process and soil biodiversity. This requires the adoption of management practices that enhance soil biological activity and build-up long term soil productivity and health. 

The main practices to enhance soil fertility include the use of organic fertilizers such as (Figure 7-4): 

A. Compost and vermicompost 

B. Green manures 

C. Animal manure 

D. Microbial fertilizers 

E. Mineral fertilizers 

A.Composting

Composting is the process of transforming organic materials of plant or animal origin into humus in heaps or pits. Compared with uncontrolled decomposition of organic material, decomposition in the composting process occurs at a faster rate, reaches higher temperatures and results in a product of higher quality. 

Within the process of composting, three main phases can be distinguished: the heating phase, the cooling phase and the maturing phase. However, these phases cannot be clearly separated from one another. (Figure 7-5 & 7-6 and Table 7-1). 

1.THE HEATING PHASE: 

• Within 3 days of setting up the compost heap, the temperature in the heap rises to 60 to 70 °C and usually stays at this level for 2–3 weeks. Most of the decomposition occurs during the heating phase. 

• In this phase, it is mainly bacteria which are active. The high temperature is a result of energy released during conversion of easily decomposable material by the bacteria. The warm temperature is a typical and important part of the composting process. The heat destroys diseases pests, weed roots and seeds. 

• During this first phase of the composting process, the bacteria have a very high oxygen demand due to the rapid development of their population. High temperatures in the heap signal that there is an adequate supply of oxygen for the bacteria. If there is not enough air in the heap, bacterial development will be hindered and the compost will develop an unpleasant odour. 

• Humidity is also essential to the composting process, as bacteria require humid conditions for their work. The need for water is greatest during the heating phase because of high biological activity and strong evaporation occurring during this phase. 

• As the heat increases, the pH of the compost heap rises (i.e. acidity decreases). 

2.THE COOLING PHASE: 

• Once the material which is easily digested by the bacteria has been converted, the temperature in the compost heap declines slowly and will remain at 25–45 °C. 

• With the decline in temperature, fungi settle and start the decomposition of straw, fibres and wooden material. As this decomposition process is slower, the temperature of the heap does not rise. 

• As the temperature drops, the pH of the composting material declines (i.e. acidity increases). 

3.THE MATURING PHASE: 

• During the maturing phase nutrients are mineralised and humic acids and antibiotics are built-up. 

• Red compost worms and other soil organisms start to inhabit the heap during this phase. 

• At the end of this phase the compost has lost about half of its original volume, has the colour of dark, fertile soil and is ready to use. 

• The longer it is stored from now on, the more it loses its quality as a fertilizer, while its capacity to improve soil structure increases. 

• In the maturing phase, the compost needs much less water than in the heating phase. 

4. DIFFERENT SYSTEMS AND METHODS 

Compost systems can be divided into continuously-fed and batch-fed systems: 

• Continuously-fed systems: These systems do not heat-up during the composting process. They are handy if there is a continuous supply of wastes (e.g. kitchen waste). However, they lack the advantages of the heating phase. 

• Batch-fed systems (all material is composted at once): These systems lead to a hot composting process. They offer the advantages of reduced nutrient loss death of weed seeds and diseases as a result of the high temperature of composting, the process is fast (within a few weeks) and it results in a compost of superior quality. If little water is available, composting in pits may be more appropriate since humidity is conserved better in pits than in heaps. 

• Vermi-composting: is a method of composting using earthworms. Earthworms speed up the composting process, aerate the organic material and enhance the finished compost with nutrients and enzymes from their digestive tracts. Vermicomposting allows you to create compost round the year, indoor during the winter and outdoor during the summer (Figure 7-7). 

B. Green Manures

Green manures are plants grown to accumulate nutrients for the main crop. When they have built up maximum biomass, they are worked into the surface soil. As they are usually cut before flowering, growing a green manure is thus different from growing a legume crop in the rotation. Once worked into the soil the fresh plant material releases nutrients quickly and will be fully decomposed within a short period of time. Old or coarse material (e.g. straw, twigs, etc.) will decompose at a slower rate than fine material and will therefore contribute more to the build-up of soil organic matter than to fertilizing the crop. 

An alternative to sowing a green manure crop in the field is to collect fresh plant material from elsewhere and work it into the soil. For example, trees and/or shrubs growing alongside crops in an agroforestry system may provide a large quantity of green material which can be used as green manure or for mulching (Figure 7-8). 

1.GREEN MANURES HAVE A NUMBER OF BENEFITS: 

• They penetrate the soil with their roots, make it more friable and bind nutrients, which would otherwise be washed away. 

• They suppress weeds and protect the soil from erosion and direct sunlight. 

• If legume plants are used, nitrogen is fixed from the air into the soil. 

• Some green manures can be used as fodder plants or even to provide food for human consumption (e.g. beans and peas). 

• By decomposing, green manures release all kinds of nutrients in the correct mixture for the main crops to utilise thus improving their yield. 

• The incorporated plant material encourages the activity of soil organisms, and builds up organic matter in the soil. This improves soil structure and water holding capacity. 

Green manuring is thus an inexpensive way to improve soil fertility and the nutrition of the main crops grown. 

2.FACTORS TO CONSIDER BEFORE GROWING GREEN MANURES: 

• Labour is required for tillage, sowing, cutting and incorporation of plants into the soil, and is most intensive where the amount of helpful equipment available is small. 

• If green manures are intercropped with the main crops, they compete for nutrients, water and light. 

• When old or coarse plant material is incorporated into the soil, nitrogen may be temporarily immobilised and therefore unavailable for plant growth. 

• If food and space are in short supply it may be more appropriate to grow a food crop rather than a green manure and recycle the crop residues, or to intercrop a green manure crop with the main crop. 

• The benefits of green manures occur over the long term and are not always visible immediately. 

3.HOW TO USE GREEN MANURES 

a) Sowing the green manure 

• If grown within a crop rotation, the time of sowing must be chosen such that the green manure can be cut down and worked into the soil before the next crop is sown. 

• Green manures need water for germination and growth. 

• The ideal seed density must be tested for each individual situation. It depends on the species chosen. 

• In general no additional fertilization is necessary. If legumes are grown in a field for the first time, inoculation of the seeds with the specific rhizobia may be necessary to profit from nitrogen fixation of the legume. 

b) Working the green manure into the soil (Figure 7-9) 

• Timing: The time gap between digging in the green manure and planting the next crop should not be longer than 2 to 3 weeks, so as to prevent nutrient losses from the decomposing green manure. 

• Crushing: Green manures are worked in most easily when the plants are still young and fresh. If the green manure plants are tall or contain bulky and hard plant parts, it is preferable to chop the plants into pieces to allow easier decomposition. The older the plants, the longer decomposition will take. The best time to dig in green manure plants is just before flowering. 

• Depth of incorporation: Green manures should not be ploughed deeply into the soil. Instead they should only be worked in to the surface soil (in heavy soils only 5 to 15 cm deep, in light soils 10 to maximum 20 cm deep). In warm and humid climates the material can also be left on the soil surface as a mulch layer. 

c) How to choose the right species? 

There is a large variety of plants, especially legumes that can be used as green manure crops. It is important that appropriate species are chosen. Most importantly they should be adapted to the local growing conditions, especially rainfall and soil, fit into the crop rotation and not pose a risk of transmitting diseases and pests to other crops (Figure 7-10). 

C.Animal Manure

Depending on whether animals are kept in stables or not (part or full time), farmyard manure consists of animal excreta and bedding material (usually straw or grass). Farmyard manure is extremely valuable organic manure. 

Some characteristics and effects of farmyard manure: 

• It contains large amounts of nutrients. 

• Only part of the nitrogen content of manure is directly available to plants, while the remaining part is released as the manure decomposes. The nitrogen in animal urine is available in the short-term. 

• When dung and urine are mixed, they form a well-balanced source of nutrients for plants. The availability of phosphorus and potassium from farmyard manure is similar to that from chemical fertilizers. Chicken manure is rich in phosphorus. However, it is important to be aware of the origin of the manure, as chicken manure from conventional farms is contaminated by heavy metals. 

• Organic manures contribute to the build-up of soil organic matter and thus improve soil fertility. 

How to Store Animal Manure

Farmyard manure should ideally be collected and stored for a while so as to obtain a manure of high quality. The best result is achieved if the farmyard manure is composted. Manure stored under anaerobic conditions (e.g. in water logged pits) is of inferior quality. 

Collection of farmyard manure is easiest if the animals are kept in stables. For storage, the manure should be mixed with dry plant material (straw, grass, crop residues, leaves etc.) to absorb the liquid. Straw that has been cut or mashed by spreading it out on a roadside can absorb more water than long straw. 

Usually, the manure is stored next to the stable, either in heaps or in pits. It can also be stored within the stable as bedding, provided it is covered with fresh bedding material. In any case, the farmyard manure should be protected from sun, wind and rain. Water logging, as well as drying out should be avoided, so as to avoid nutrient losses. The storage site should be impermeable and have a slight slope. Ideally, a trench collects the liquid from the manure heap and the urine from the stable. A dam around the heap prevents uncontrolled in- and outflow of urine and water. 

Storing manure in pits is particularly suitable for dry areas and dry seasons. Storage in pits reduces the risk of drying out and the need to water the pile. However, there is greater risk of waterlogging and more effort is required as the pit needs to be dug out. For this method, a 90 cm deep pit is dug with a slight slope at the bottom. The bottom is compressed and then first covered with straw. The pit is filled with layers about 30 cm thick and each layer compressed and covered with a thin layer of earth. The pit is filled up until it stands about 30 cm above ground and then covered with 10 cm of soil. 

Humidity in the manure heap must be controlled. To avoid nutrient losses, it should neither be too wet nor too dry. Some indicators to monitor the humidity of the manure are (Figure 7-11): 

• If white fungus appears (threads and white spots), the manure is too dry and should be dampened with water or urine. 

• A yellow-green colour and/or bad smell are signs that the manure is too wet and not sufficiently aerated. 

• If the manure shows a brown to black colour throughout the heap, the conditions are ideal. 

D. Microbial Fertilizers

The microbial fertilizers mostly consist of organic material and some source of sugar or starch, which are fermented together with specific species of microorganisms. The products are living organisms and need to be applied cautiously. They should not be used when expired, since the organisms may be dead. 

Although some research has been done on the use of microorganisms and positive effects may be proven, there is still little experience with such products. To find out the effect of a certain product, it is recommended to test them in small scale and compare with an untreated plot. Remember though: microbial fertilizers cannot substitute an appropriate humus management in the farm. 

Most of the bacteria and fungi present in the purchased products are generally present in soil. Microbial inocula, therefore, enhance the presence of the specific organisms. Some farmers make their own microbial fertilizers to save on costs (Figure 7-12). 

Some microbes add nutrients to the soil through mineralisation. Others add nitrogen by fixing it from the atmosphere. These include Rhizobium and Azotobacter. Other microbes, such as Mycorrhizal fungi, help to supply plants with phosphorus. Azospirillum and Azotobacter are bacteria that can fix nitrogen. Pseudomonas species are a diverse group of bacteria that can use a wide range of compounds that plants give off when their roots leak or die. They are able to solubilize phosphorus and may help to suppress soil borne plant diseases. 

E.Mineral Fertilizer

The mineral fertilizers, which are allowed in organic agriculture, are based on ground natural rock. However, they may only be used as a supplement to organic manures. If they contain easily soluble nutrients, they can disturb soil life and result in unbalanced plant nutrition. In some cases, mineral fertilizers are ecologically questionable as their collection and transport is energy consuming and in some cases natural habitats are being destroyed. 

You can find in further reading the list of approved substances for use in soil fertilizing and conditioning (from the CODEX 2013). 

REFERENCES 

Codex 2013, Guidelines for the production, processing, labelling and marketing of organically produced foods, Codex Alimentarius Commission – FAO, WHO 

IFOAM. 2003. Training Manual for Organic Agriculture in the Tropics. Edited by Frank Eyhorn, Marlene Heeb, Gilles Weidmann, p 66-77, 124-150, http://www.ifoam.bio/ 

Shiva V., Pande P., Singh J. 2004. Principles of organic farming: Renewing the Earth’s harves. Published by Navdanya, New Delhi, India. 

SOURCES 

IFOAM 

ON TECA 

Nutrient Management in Organic Agriculture: http://teca.fao.org/read/8368