Alley cropping in the tropics

The problem of compaction within annual crop systems is a very difficult one in the tropics especially. The tropics make it difficult because they naturally lend themselves to diverse productive tree systems and often people have followed the annual cropping systems of European style of cultivation.

Ok so incorporating trees in the systems will help and mulching definitely will help and limiting the amount of walking on the fields will help. What I have found is because most of these people are used to burning mulch they cut at the end of the wet when it is still green and easy to cut and burn the same mulch in the dry season after it is dried right out and so easy to burn.

Then when you ask them to try using mulch they feel a bit weird and don’t want to be laughed at by other farmers who usually think the mulch looks untidy when their fields are all clean bare soil (a power trip over nature, control issue, and they also have the old idea backing them up “cleanliness is next to Godliness”). So when you eventually persuade them to try mulching a few problems usually arise unless you follow it right through with, and believe me these have been very hard and at times very disappointing mistakes to make.

Number one: local people usually do not want to look anymore different and get laughed at more so they cut their mulch at the same time of year as everyone else, this is a MAJOR mistake as the mulch lies in the full dry season sun with most of the moisture evaporating and with very limited soil life activity in the dry hot conditions and by the time the wet season starts again most of the benefit of the mulch is gone. You need to cut the mulch at the end of the dry at the beginning of the wet when the cloud cover increases and the evaporation effects go down.

This means that the shade from the trees is opened up to grow the crops and the ground is protected from the heavy rain reducing erosion and leaching effects of soil nutrients and the valuable manure fertilizer additions. The mulch also starts to break down quicker as the soil life activity is considerably higher in the rainy season with the extra soil moisture and lower soil temperatures, which means it becomes soil food, humus and potential plant food much faster also increasing the soils organic matter content and soil water holding capacity.

This means we can select legume trees which can be pollard cut every year, planted on contour in rows within the crop fields spaced so that all the mulch cut from the trees mulches the crop rows on contour (often only a double reach row in width so the crop can be attended to from either side of the row without walking on the soil between the crop plants) between the tree rows, Gliracidia and Leaucaena are 2 very typical trees used for this purpose.

They will also have the beneficial effect of structuring the soil and fixing large amounts of nitrogen in the root zone and because they are pollard cut at the start of the rains to a tall stump (say 1.8m) they create very little shade during the main growing season. By the time the rainy season finishes they will have re-sprouted multiple long thin branches that will quickly leaf up and shade the ground during the dry season, this shading effect is more beneficial in the dry than mulch and as the trees have deeper roots than the annual crops the continue to grow on the deeper residual ground water well after the rainy season has finished. With the ground well shaded and the mulch from the pre-rainy season mulch cut well rotted into soil humus it is often possible to sow a dry season cover crop in the field as and extra soil conditioner, mulch harvest for the start of the next rainy season. After a few years of establishment of this system and the inevitable improvement of soil structure, organic matter and water holding capacity, it with be possible to take off some surplus legume tree pruning leaf material to feed animals and returning the animals manure to the field as fertilizer and take off surplus legume tree pruning branch wood material to use as cooking fuel, breaking the dangerous soil nutrient depleting cycle often practiced of burning animal manure for cooking fuel.

Number two, when mulch is cut to burn it is cut in the form of large pruning material and left propped up to quickly air dry so it will burn easily, once this much material has dried in long lengths it is very hard and brittle making it a great deal harder to chop up. But this is what is often done because local people don’t want to look TOO different in case it does not work. To get the most benefit from the mulch we need to chop it up and greatly increase its surface area and its capability to lie in contact with the ground so that beneficial soil fungi and bacteria can begin to work on it as quickly as possible. To get the most benefit from our effort we should cut the pruned mulch material up as small as practically possible (often a minimum of 150mm lengths using hand tools) while it is still green and containing its natural plant moisture to help increase the speed of its decomposition rate.

Number three, not enough mulch is added usually because local people are caution about doing something so different again. The mulch needs to be thick at least 100 to 150mm so that it has an intrinsic ability of holding moisture and also insulating the soil, which will require one tenth the amount of water once the thick mulch is initially soaked than bare soil. The subtle but precise placement of the mulch is often not taken seriously enough either. Initially the majority of the pruned mulch material should be piled in long rows close to the legume tree trunks, this will help relieve the stress on these useful functional elements of the system and help them recover. Between the pruned trees where the crop is seeded only a very thin mulch is required on top of the organic fertilizer that is being used (usually manure or ideally compost), a “germination mulch” just enough to completely cover the ground and germinate the crop seed but not TOO much to inhibit the crop seed from germinating, with a little bit of trial and error this measure of mulch is easily gauged.

Once the crop has germinated and has started to grow and as it is attended to the farmer can walk along next to the pruned legume trees on top of the long mulch pile greatly reducing the effort of compaction. As the crop begins to increase in size more and more mulch can be moved across from the tree lines to the soil surface around the base of the crop plants until at the end of the growing season the mulch situation will have completely reversed and the majority of the mulch will be around the crop plants and only a small amount on the tree lines. At the end of the cropping season the crop plants themselves can be cut and all unusable parts returned as mulch leaving their roots in position to root in position to decompose forming compost corridors in the soil enhancing soil structure further. At this stage chickens can be ranged through the field to evenly spread and further shred the mulch, eat weed seeds and insect pests and their larvae which are often dormant in the top soil at the end of the cropping season, this helps break the weed and pest cycles.

Increasing the speed of mulch decomposition because of the shredding and manure drop in turn increasing the fertility of the field and gives the chickens a good nutritious feed. When the chickens have turned over most of the mulch they should be removed and a dry season cover crop seeded for cutting as mulch at the beginning of the next wet season. At this stage chickens can be ranged through the field again to spread the mulch eat weed seeds, insects and deposit manure for a short period just prior to seeding the main crop and cutting the main legume tree mulch.