Feeding the soil for climate resilient coconut farms
By Winfried Scheewe
Severe weather events triggered by climate change such as drought are expected to increasingly affect coconut production. As explained in the articles Coconut farms and climate change: what is at stake and what can be done?, and Coconut palms under climate stress, the principal key to enabling coconut palms to better cope with heat and water stress lies in the soil. In this article, we will discuss several practices which can help to restore, improve and maintain fertile and resilient soils in coconut farms.
In view of the risks of drought and extreme rainfalls, the foremost aim is to enhance the water holding capacity of the soil, so that the coconut palms and intercrops can cope more effectively with dry spells and drought as well as with flooding.
These practices aim not only to improve the soil but should also make the whole farm resilient in the face of climate change and the projected increase of extreme and variable weather. Moreover, all options will also help farmers to improve overall farm productivity under normal conditions.
When we wish to improve the health of the soil in coconut farms, we may consider the insights from soil ecology discussed in the article The Coconut Farm Below Ground – Appreciating Vital Processes in the Soil:
- Organic matter needs to be recycled.
- Soil needs to be covered to protect the organisms (and the processes in soil) against heat and rain and to avoid erosion. Therefore, the use of herbicides should be avoided.
- There should always be a diversity of deep rooting plants.
- Processes in the soil should not be disturbed by plowing and the use of herbicides as these practices remove the protection against heat and erosion. This leads us to the question: What can we do to increase and maintain a favorable level of soil fertility? Fortunately, farmers have several options to improve and maintain soil fertility in coconut farms. Among them are (a) mulching as a way to recycle organic matter, (b) planting of green manure and cover crops, as well as (c) the planting of hedgerows of nitrogen fixing shrubs.
Mulching
Mulching is the most natural way to recycle organic matter. The covering of the soil with fronds, and when suitable, husks or other plant residues, especially around the trunk of the palm trees, helps the soil absorb more rain water. The runoff of water during heavy rains, and by this the risk of soil erosion, is significantly reduced. Mulch also conserves soil moisture as evaporation is reduced and hampers weed growth. The mulch layer also protects the soil organisms which improve the soil’s sponge like structure.
As the mulch layer gradually decomposes, the additional humus augments the soil acidity. The pH level improves towards neutral and some nutrients, especially micro-nutrients, become more accessible to the crops. Likewise, the decomposed organic matter also provides energy to nitrogen-fixing bacteria active in the soil. The improving soil structure allows roots to develop better.
We need to consider the height of the mulch layer. If the layer is too thick, rats might be attracted to breed there. Thick layers of decomposing organic material may also invite female rhinoceros beetles to lay their eggs there. It is better to keep the layer at lower height while adding new layers of organic matter from time to time.
Coconut husks are very valuable to cover soil’s surface and to protect the root of trees. Many farmers place coconut husks around the stems of intercropped trees. Husks should be laid with the open side down to avoid the impounding of water and breeding of mosquitoes. Slashed weeds and shrubs are usually left on the ground to decompose but they can also be utilized to cover the roots of palms or the soil among intercrops.
In view of the many positive effects of mulching, burning of crop residues and other organic materials should be avoided. Similarly, the use of herbicides would deny the soil the positive effects of the slashed and mulched vegetation.
Green Manure and Cover Crops
Farmers may also consider replacing nonproductive grasses and shrubs with cover crops, which will add more organic matter to the soil and cover it efficiently. The practice of cultivating specific crops just for improving the soil dates back to the Roman Empire more than two millennia ago when Cato the Elder advocated for the use of manure and green manure as fertilizer.
Cover crops are commonly leguminous plants which make atmospheric nitrogen available to the soil. While shading the soil, cover crops also add organic matter to the soil, thereby improving topsoil depth, water-holding capacity, nutrient content, friability, and texture.
Cover crops require no capital outlay after the initial purchase of seed. To enhance organic matter production, farmers may occasionally press the vegetation down with a roller. This stimulates the growth of new leaves and branches. Several cover crops have been tested in coconut farms, such as:
Calapogonium is a vigorous, creeping, and twining hairy herb forming a tangled mass of foliage 30 to 40 cm deep. Stems are succulent, covered with long brown hairs, creeping in lower parts, with roots at nodes which come in contact with the soil and the upper part of stem twining. The leaves are trifoliate and the leaflets are hairy on both surfaces. Its flowers are blue with greenish-yellow blotches. Its seed pods are linear, 2.5 to 4 cm long, and are yellowish brown. The pods contain four to eight-seeds and are densely covered with long erect hairs.
Tropical kudzu (Pueraria phaseoloides) has been extensively introduced in tropical and subtropical areas to be used as a forage and soil improvement species, and it can be found naturalized throughout the humid-tropics. It is often used as a nitrogen-fixing green manure or grown as a cover crop in coffee, oil palm, citrus, and rubber plantations. Its leaves are similar to those of the calopogonium in shape, though are a bit larger. Tropical kudzu leaves and stems are rich in protein (about 19%) and are palatable to livestock, and so are used widely as animal fodder. It may also be used as a pasture crop when grown with a suitable grass such as Guinea grass (Panicum maximum). The very deep and extensive root system not only provides edible tubers, but also helps minimize soil erosion. The plant’s rapid growth also helps suppress weeds and protect the soil from rapid run-off water.
Kudzu is a vigorous, perennial climbing plant producing annual stems up to 5mm in diameter and 2 - 10 meters long from a tuberous rootstock. These stems scramble over the ground, where they produce new roots at the nodes, and also twine into the surrounding vegetation for support.
Centrosema pubescens is an evergreen perennial climber that grows at a fast rate. It is suitable for most soils and can even grow in nutritionally poor soil. It is suitable for mildly acidic, neutral, and mildly alkaline soils. It can grow in full shade, semi-shade, or no shade. Centrosema pubescens prefers moist or wet soil and can tolerate drought.
Velvet bean is a vigorous, nitrogen-fixing, bushy or vining annual plant. The vines will climb anything available and can reach 10 m or more in length. The velvet bean is somewhat drought tolerant and is an excellent green manure or cover crop, with the ability to suppress weeds and provide generous amounts of organic matter and nitrogen to the soil.
Arachis pintoi, or false peanut, can also serve as a cover crop. It has high forage quality and is used as ground cover in many fruit orchards. Like camote (sweet potato) it is readily propagated from cuttings.
The above-mentioned plants have been promoted for many years as cover crops. After the cover crop has established itself, it can be slashed and food crops can be planted, or it can be left permanently. One way to increase the production of biomass is to press down the mature foliage with a heavy roller to initiate new sprouts of the cover crop. Care must be taken as some of the cover crops tend to climb on smaller trees and younger palms and could potentially hamper their growth.
Like mulch from plant residues, cover crops help control grass and weeds. Most cover crops can be used to suppress cogon grass and similar weeds. In addition, they have deep roots which bring nutrients from deeper sections to the topsoil and improve the soil’s water holding capacity.
Hedgerows of nitrogen fixing shrubs
Several farmers in Mindanao have experiences with planting hedgerows of nitrogen fixing shrubs such as Flemingia macrophylla, Desmodium rensonii, or Indigoferra anil near coconut palms. Scientist in Sri Lanka have frequently explored the planting of Gliricidia sepium (kakawate, madre de cacao) and Leucaena leucocephala (ipil-ipil) to increase the available organic material (biomass) by using the prunings (twigs and leaves) as mulch for coconut palms.
The shrubs can be established in rows parallel to the palms. This practice is an adaptation of the Sloping Agriculture Land Technology (SALT) chiefly developed to protect and improve upland soil. SALT suggests the planting of double hedgerows on slopes about four meters apart. After the shrubs are well established (after about six months), they are regularly slashed (for example before the harvest of coconuts) and the branches are used for mulching. Some farmers have planted dense hedgerows left and right of each row of coconut palms.
Ideally, each hedgerow consists of a double-row of nitrogen-fixing trees spaced forty to fifty-centimeters apart, while the double hedgerow is about two meters apart from the palms in order not to compromise the roots of the palms. Another option is to plant the nitrogen-fixing trees in circles around the palms.
Farmers preferably prune the shrubs at least quarterly at a height of about one meter. The trees regrow faster if about 10 percent of the leaves are left. The cut branches are placed around the trunks of the palms to cover their roots. This method helps to improve the soil within one year and leads to a significant yield increase in the second year.
Nitrogen-fixing shrubs continuously produce organic matter over several years without the need for replanting. These small trees can be considered nutrient pumps. The straight roots absorb nutrients from deeper soil layers, which are released when leaves and branches decompose on the surface. By this, the nutrients are made available to other plants. It should be noted that no or little competition with palms exists due to the vertical roots of the shrubs.
While adding biomass for mulching, the growth of coconut roots is enhanced and the capacity of the soil to absorb and hold water is significantly increased. Compared to cover crops, nitrogen fixing shrubs can be combined with intercrops analog to the SALT system.
How to plant hedgerows? The ideal way is to plant double rows left and right of the palms. It is recommended to space the double hedgerows 50 cm apart. The seeds are planted in shallow furrows and are slightly covered. Depending on the kind of shrub, one kg of seeds can plant up to 100 meters of hedgerow. Costs for seeds are approximately PhP 500/kg. To gain experience, it is advisable to start with a small area, perhaps one fourth of a hectare. Also, different kinds of shrubs may be tested on a small scale. Seeds from the first shrubs can be collected for further propagation.
The above-mentioned practices, which can also be combined in various manners, allow coconut farmers to adapt to the challenges related to climate change so that life, property, and income of individuals can be protected and productivity can be enhanced. When coconut farmers improve soil fertility and increase diversity by planting trees and shrubs, they also help move carbon dioxide (CO 2 ) from the atmosphere–where there is too much of it–into the soil, where it becomes soil carbon and enhances climate resilience. In other words, by protecting coconut palms and other crops against severe weather events, farmers also help to mitigate (or lessen) CO 2 in the atmosphere, which causes climate change.
The content of this article relates information covered by the curriculum Climate Resilient Coconut Farm Management designed for a three-day farmers training. The curriculum is available at: https://snrd-asia.org/climate-resilient-coconut-farm-management/
The curriculum has been prepared by the GIZ Responsible Land Governance Mindanao (RLGM) Project in cooperation with the Gingoog Bay Alliance (GBA) and was tested end of 2020/early 2021.
Photos by Winfried Scheewe