Biological ways to increase yield and improve the soil

FOOD CROPS. Biological ways to increase yield and improve the soil. Chapter 11 in the book CIAT / Reinhardt Howeler 2014. Publication No. 389; Reference manual: Sustainable Soil and Crops Management of Cassava in Asia  280 p.

                                                  (Summary for Chapter 11)

In many parts of the world, crops are still grown without application of chemical fertilizers, either because they are not available or they are considered too costly. In that case, farmers often try to maintain soil fertility through various biological means, including shifting cultivation, agro-forestry, crop rotation, green manuring, mulching, cover cropping, alley cropping, and intercropping, as well as the application of animal manures or compost. In general, these methods are most suitable in areas where labor is available and cheap, and where purchased inputs like fertilizers are unavailable or expensive, particularly in extensive agriculture systems. They can also be used to supplement the application of chemical fertilizers, mainly to increase the organic matter content of the soil, which will improve soil structure, bulk density, soil aggregate stability, water- and nutrient-holding capacity, and drainage.   

It can be concluded that cassava is a very weak competitor and suffers serious setbacks if it has to compete with weeds, intercrops, or cover crops, especially at the early stage of establishment, due to its slow initial rate of growth. 

Specific observations and recommendations on how to increase cassava yields and improve soil fertility through biological means can be summarized as follows:  

Intercropping. From these various intercropping experiments we can conclude that, when well-managed, intercropping with short-duration food crops will generally result in reduced soil loss by erosion due to more rapid canopy development. Incorporation of the intercrop residues will tend to increase soil organic matter and, if the intercrop is a legume, may contribute more N fixed from the air.  But to maintain high yield of both cassava and the intercrops, both crops should be adequately fertilized.  

Applying commercial fertilizers in combination with animal manures or compost. For that reason, it is recommended to apply commercial fertilizers in combination with animal manures or compost; and, if diseases and pests are not a major problem, to incorporate back into the soil all the cassava crop residues in order to return  organic matter and plant nutrients. 

Alley cropping. Among biological solutions mentioned above, alley cropping between leguminous hedgerow species seems to have the greatest long-term beneficial effect on cassava yields and soil fertility. Once established, the hedgerows require little maintenance besides regular pruning and they can survive for at least 15–20 years without the need for replanting. Besides improving soil fertility, the prunings, when mulched on the soil surface, will help to control weeds and erosion, reduce soil surface temperatures, and increase soil moisture.   

Cover crops: Most perennial cover crops will strongly compete with cassava at the early stages of growth, resulting in low cassava yields. Most intercropped green manures or long-duration intercrops will also tend to reduce cassava yields;  

Green manures. Most beneficial are some of the green manures when they are grown and incorporated before planting cassava, but only in areas with a long wet season that provides sufficient soil moisture during most of the cassava growth cycle.  

Mulching. Similar beneficial effects of mulching have also been obtained when native weeds were cut and mulched before planting cassava with minimum tillage. 

Shifting cultivation.  In the past, shifting cultivation was practiced when land was plentiful and no other alternatives existed for restoring fertility after several cropping cycles. However, in most countries land is now scarce and not enough land is available for the long fallow periods necessary to maintain soil fertility;  

The keys biological solutions. If no chemical fertilizers are available or are too costly, soil fertility can at least be partially restored by some of the biological solutions discussed in this chapter. 

CHAPTER 11

BIOLOGICAL WAYS TO INCREASE YIELD AND IMPROVE THE SOIL

...

Shifting cultivation

In many areas in the tropics, farmers try to maintain soil fertility through shifting cultivation, also known as “slash-and-burn” systems. After several years of cropping, the land is left to return to bush fallow or forest for 10–20 years, to let the soil rest and replenish nutrients lost during the cropping cycle. However, because of rapid population growth and the consequent increase in land pressure, the fallow period has steadily been shortened while the cropping cycle and intensity have increased.

Research conducted on very poor and degraded soils in southern Colombia indicates that even long periods of bush fallow were not able to fully restore soil fertility, and cassava yields remained below 8–10 t/ha. By contrast, with the application of N, P, and K in chemical fertilizers, cassava yields doubled or tripled, reaching over 24 t/ha in the third consecutive planting. In this and many similar situations, farmers could greatly increase their income if they would grow cassava on a more permanent basis on the best and flattest land using commercial fertilizers, while leaving the steeper and more degraded fields in permanent pasture or planting coffee, fruit trees, or forest.

In addition, when slash-and-burn systems are practiced on steep slopes, such as in Lao PDR, and parts of western Vietnam, after burning of the forest during the dry season, much of the resulting ash is washed down-slope with the first rains of the wet season before crops can be planted, making the system ineffective in replenishing soil fertility. The result is a steady decline in soil fertility, an increase in soil erosion, and decreasing crop yields.

Alley cropping

This is a type of agro-forestry in which crops and trees are combined in the same field with the objective of improving the nutrition of the crops, and in some cases to reduce erosion.  Crops are planted in the alleys between rows of fast-growing leguminous trees. The space between hedgerows can be varied, but is usually around 4–5 meters, so that less than 20% of the total land area is occupied by the hedgerows.

The trees are cut back at least once a year to about 50 cm above the ground and the prunings are either incorporated into the soil of the alleys before the crop is planted, or mulched on the soil surface to supply nutrients and to control weeds and soil erosion.

The benefit of leguminous trees is that they can fix considerable amounts of N, which is added to the soil in the alleys through the decomposing prunings. In addition, the trees are deep rooted and are able to take up nutrients from deeper soil layers and recycle them to the top soil, where they become available to the crops.

Also, because the trees are deep rooted they compete less for water and nutrients than fast growing intercrops. The trees will need to be pruned regularly, but do not require replanting for many years, and therefore do not require the annual purchase of seed.

The results of several alley-cropping experiments can be summarized as follows:

·         Best results have been obtained with the leguminous shrub or tree species of Leucaena leucocephala, Gliricidia sepium and Flemingia macrophylla.

·         The trees are planted in rows about 4–6 meters apart and several rows of cassava are planted in the space (alleys) between the rows of trees.

·         The trees need to be cut back to about 50 cm above the ground at least once a year, before planting cassava, and the prunings are incorporated into the soil of the alleys before planting cassava, or are spread as a mulch on top of the soil.

·         Cassava yields may or may not increase for the first few years of cropping as the trees take time to establish. But after a few years, cassava yields will increase, soil loss by erosion will decrease, and soil fertility will markedly improve.

·         Mulching or incorporating the tree prunings will increase the organic matter content of the soil and improve the soil’s chemical and physical characteristics.

·         The three tree species mentioned above will not need to be replanted for many years, but the leguminous shrub, Tephrosia candida, will need to be replanted every 3–4 years

Cover cropping

Cover crops are usually perennial forage legumes planted to fix N and recycle soil nutrients to improve soil fertility, and to prevent serious soil erosion on sloping land. Annual crops may be planted in individual planting holes or in strips where the cover crop has been incorporated into the soil or killed with herbicides.

Several cover-cropping experiments, conducted in Colombia and Thailand, show that cassava is a weak competitor and that yields are reduced markedly if plants have to compete with deep-rooted and well-established forage legumes used as cover crops. This competition is particularly strong during cassava plant establishment, especially when this coincides with a period of drought. Thus, cover cropping with most forage legumes is not practical since it tends to reduce cassava yields and requires considerable additional labor.

Green manuring

This usually refers to the practice of growing a grain or forage legume on the land for several months prior to planting the main crop. The green manures are generally cut after 2–3 months of growth and are either incorporated into the soil or mulched on top of the soil before planting the main crop. This will improve the soil’s fertility, especially that of N, due to N fixation by the legumes. However, green manures can also be planted as an intercrop within the main crop and slashed back and mulched after 2–3 months of growth; or they can be planted as narrow strips alternating with strips of the main crop.

Many green manure species have been tested to see their effect on the following cassava crop. In a highly acid soil in Colombia, green manures were incorporated into the soil before planting cassava. From this experiment, it can be concluded that cassava yields increased most by the application of fertilizers, but incorporation of green manures also helped to increase yields, especially when no fertilizers were applied to cassava. Peanut was among the most effective species, but Zornia latifolia, Pueraria phaseoloides and Centrosema pubescens were also very effective, especially in the presence of fertilizers.

 

Other experiments, conducted on sandy and highly infertile soil on the north coast of Colombia, suggest that among green manures tested, the mulching of Canavalia ensiformis and native weeds were most effective, while Crotalaria juncea was least productive and least effective in increasing cassava yields. 

In India, the standard recommendation for cassava is to apply 100 kg N, 50 kg P₂O₅ and 100 kg K₂O/ha as chemical fertilizer, together with 12.5 t/ha of farmyard manure (FYM).  Since FYM is expensive and cumbersome to transport and apply, a long-term experiment was conducted from 1990 to 2004 to determine whether green manuring with vegetable cowpea could reduce the need for FYM and/or reduce the high levels of chemical fertilizer input.

Vegetable cowpea was planted during pre-monsoon rains in February and after the harvest of green pods, the total crop biomass was incorporated into the soil before planting cassava in May. The effect of incorporating the crop residues of cassava back into the soil after harvest was also investigated. The results indicate that by practicing green manuring with cowpea biomass, the application of FYM as well as that of N and P could be reduced to only 50% of the previously recommended rates. Also, the annual incorporation of cassava crop residues could completely replace the application of 12.5 t/ha of FYM as long as the recommended rates of N, P, and K were applied.

Many green manure experiments have been conducted in Thailand to determine the most effective green manure species and their management for the local climatic and soil conditions. In the cassava-growing areas of Thailand, total annual rainfall is about 1,200 mm, with the rainy season starting in about May and terminating in October.

In one experiment, conducted for five years, three green manures were planted in the very early part of the rainy season and their biomass incorporated into the soil after 60 days of growth, after which cassava was planted and then harvested after 10 months. It was found that cowpea was more effective than Crotalaria juncea in increasing cassava yields, while pigeon pea had little beneficial effect. Cowpea produced more biomass, and thus had a higher nutrient content. In addition, it improved the physical conditions of the soil, such as bulk density and water infiltration rates.

Other experiments conducted in Pluak Daeng, Thailand, concluded that, among the green manures tested, Crotalaria juncea was the most productive and the most effective in increasing cassava yield; that incorporation of the green manures resulted in slightly higher cassava yields than mulching; and that some green manures were as effective as – or even more effective than – chemical fertilizer in increasing cassava yield. However, under the climatic conditions of Thailand, which has a 6-month dry season, the traditional use of green manures is impractical, since the better part of the relatively short rainy season is used for the production of green manures, while the following cassava crop produces low yield due to drought stress in the dry season. For that reason, green manuring is seldom adopted by Thai cassava farmers.

Another alternative is to plant the green manures in the early part of the rainy season, cut them back and mulch the biomass after 3–4 months; then plant cassava without further land preparation and leave the crop for 18 to 21 months. This can double cassava yields and reduce production costs as the land needs to be prepared only once every 2 years, and weeding and harvesting costs are also reduced.

From these various green manure experiments the following may be concluded:

·         Planting green manures can increase cassava yields in areas with a relatively long wet season or with two short rainy seasons per year, especially when no fertilizers are applied

·         In areas with a single and relatively short wet season, planting green manures may seriously reduce cassava yields. This is because planting green manures in the early part of the rainy season markedly reduces the time that the following cassava crop can benefit from adequate rains ˗ unless cassava is left in the ground during the entire following wet season and is harvested only after18 months.

·         Intercropping with green manures at the time of cassava planting and cutting back the green manure at 2–3 months after planting (MAP) can result in low cassava yield due to excessive competition of the green manure with cassava.

·         Interplanting the green manures within a mature cassava stand at 7–8 MAP and incorporating the green manure just before the next cassava planting may increase the yield of the following cassava crop

Although green manuring may have short-term benefits for crop productivity, the long-term effects on soil fertility are not very clear. Whenever labor is scarce, farmers will probably prefer to maximize their yields through the use of commercial fertilizers.

Mulching

Mulching, that is, leaving the crop residues or other biomass on top of the soil, or bringing in such biomass from other locations, has the advantage that the mulch will reduce weed growth, preserve soil moisture, and reduce temperature fluctuations, while it also protects the soil from the direct impact of raindrops resulting in less erosion. Mulching biomass also eliminates the need for additional work incorporating the biomass into the soil. If the soil is loose the cassava stakes can be planted directly through the mulch into the soil. This method of minimum or zero tillage improves the organic matter and structure of the soil.

Results from a mulching experiment in Colombia indicate that application of large amounts (12 t/ha) of dry mulch of guinea grass (Panicum maximum) supplied the cassava plants with K, Ca, Mg, and N nutrients; helped  maintain soil moisture; and reduced the temperature of the surface soil. This resulted in increased cassava root and top biomass, increased root dry matter content while reducing its yearly variation, and decreased the cyanogenic potential of the roots, particularly in the absence of fertilizers. Over the years, both the application of mulch and that of fertilizers increased the soil P and K levels, while without mulch, the soil’s acidity increased. However, the application of such large amounts of mulch may be very labor intensive, depending on the distance between the cassava field and the source of mulch.

Crop rotation

In most countries in Asia, cassava is grown on the same fields year after year. Especially in areas with heavy soils and poor internal drainage, where root rots are frequently observed, farmers are advised to rotate cassava with other crops, especially cereals and grasses, in order to reduce the soil inoculum of the main causal agent, Phytophthora spp. With the recent appearance of witches’-broom disease, mainly in Vietnam, Thailand, and Cambodia, it is advisable to rotate cassava with other crops to prevent the spread of the disease through infected crop residues left from the previous cassava crop.

Crop rotations also can increase farmers’ income. Short-duration varieties can produce a reasonably high yield when harvested after 7–8 months, leaving enough time for another short-duration crop to be grown during the same year.

In Kerala state of India, cassava is now often grown in lowland areas where short-duration cassava varieties are planted after the harvest of a short-duration rice crop. Under those conditions, yields of cassava are substantially higher than in the traditional upland areas.  Even higher incomes can be obtained when cassava follows a crop of vegetable cowpea, or when a peanut crop follows cassava under lowland rice field conditions. However, growing cassava after a rice crop in lowland areas requires a lot of labor to build raised beds to grow the cassava without waterlogging. High cassava yields are obtained because of the higher fertility of the lowland soils as well as the higher water retention capacity of these soils during the dry season.

Researchers in Thailand have shown that rotating cassava with well-adapted grain legumes, such as peanut and pigeon pea, has long-term advantages over continuous cassava production; however, the latter is still the common practice among farmers in northeast Thailand.

Intercropping

Crops grown in association tend to cause less loss of nutrients through erosion and leaching but more loss of nutrients removed in the harvested products. Intercropping represents an intensification of the demand for nutrients, particularly when each associated crop is planted at its normal density. In this case, the removal of nutrients from the soil is higher than when cassava is grown in monoculture (Table 1).

 Table 1.   Removal of soil nutrients by the products (roots and grains) harvested in a cassava – mungbean intercropping system, compared to removal by cassava planted in monoculture.

	Nutrients removed (kg/ha)
System	N	P	K	Ca	Mg	S
Cassava in monoculture	40	5	78	19	8	6
Cassava–mungbean intercropping	90	11	84	18	10	9

A long-term intercropping experiment conducted in Thailand, showed that after 24 years of intercropping cassava with peanut or soybeans the soil organic matter content had increased from the original 1.0% to 1.2 or 1.3%, while in plots with continuous cassava monoculture it had slightly decreased to 0.9%.  In another long-term experiment conducted in South Vietnam, intercropping cassava with peanut and without fertilizer application, the soil organic matter and available P contents had increased, but the soil Ca and K contents had decreased, probably due to the greater nutrient removal by the harvest of both cassava and the intercropped peanut.

From these various intercropping experiments we can conclude that, when well-managed, intercropping with short-duration food crops will generally result in reduced soil loss by erosion due to more rapid canopy development. Incorporation of the intercrop residues will tend to increase soil organic matter and, if the intercrop is a legume, may contribute more N fixed from the air.  But to maintain high yield of both cassava and the intercrops, both crops should be adequately fertilized.

Conclusions

It can be concluded that cassava is a very weak competitor and suffers serious setbacks if it has to compete with weeds, intercrops, or cover crops, especially at the early stage of establishment, due to its slow initial rate of growth.

Specific observations and recommendations on how to increase cassava yields and improve soil fertility through biological means can be summarized as follows:

·         Most perennial cover crops will strongly compete with cassava at the early stages of growth, resulting in low cassava yields. Most intercropped green manures or long-duration intercrops will also tend to reduce cassava yields.

·         Most beneficial are some of the green manures when they are grown and incorporated before planting cassava, but only in areas with a long wet season that provides sufficient soil moisture during most of the cassava growth cycle.

·         Among biological solutions mentioned above, alley cropping between leguminous hedgerow species seems to have the greatest long-term beneficial effect on cassava yields and soil fertility. Once established, the hedgerows require little maintenance besides regular pruning and they can survive for at least 15–20 years without the need for replanting.

Besides improving soil fertility, the prunings, when mulched on the soil surface, will help to control weeds and erosion, reduce soil surface temperatures, and increase soil moisture 

·         Similar beneficial effects of mulching have also been obtained when native weeds were cut and mulched before planting cassava with minimum tillage.

·         In the past, shifting cultivation was practiced when land was plentiful and no other alternatives existed for restoring fertility after several cropping cycles. However, in most countries land is now scarce and not enough land is available for the long fallow periods necessary to maintain soil fertility. 

·         For that reason, it is recommended to apply commercial fertilizers in combination with animal manures or compost; and, if diseases and pests are not a major problem, to incorporate back into the soil all the cassava crop residues in order to return  organic matter and plant nutrients

·         If no chemical fertilizers are available or are too costly, soil fertility can at least be partially restored by some of the biological solutions discussed in this chapter.

See more ...

The future of Agriculture: Biologicals and cover crops

Eric Johnston

http://agrienergy.net/2014/11/growing-love-cover-crops/

(*) Source:  Sustainable Soil and Crops Management of Cassava in Asia hoặc tại Biological ways to increase yield and improve the soil; Cách thức sinh học để tăng năng suất và cải thiện đất; Nông nghiệp bền vững, tiếng Anh cho em.
 

Bản dịch tiếng Việt: Quản lý đất và cây trồng bền vững đối với sắn ở châu Á, sách hướng dẫn từ nghiên cứu đến thực hành. Soạn giả Reinhardt Howeler và Tin Maung Aye. Chủ biên dịch Hoàng Kim với sự cộng tác của Hoàng Long, Nguyễn Thị Trúc Mai, Nguyễn Bạch Mai. (xem tiếp CÂY LƯƠNG THỰC. Cách thức sinh học để tăng năng suất và cải thiện đất.)

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