Tropical Soils:

The tropics give us the advantage of 12-month production, however, tropical soil management is not as easy as many believe. This is because we have many imported diseases, have lost many traditional disease resistant varieties and have become dependent on commercial inputs that support disease. It is especially important to understand that tropical soils are reluctant to build organic matter as humus because nutrients are converted into plant biomass so quickly, this means that tropical soils are managed very differently than soils in temperate climates. We have all but lost the art of natural management, as heritage agriculture gives way to commercial practices requiring ever more costly inputs.

Of course, we added insult to injury as the result of introducing the practice of constant tillage, (see book ‘Plowman’s Folly’, 1943) and all the detrimental consequence of compaction and disturbance we continue to suffer from today.

Here we will continue to discuss ‘the alchemy of fertility’ for tropical soils ¬- being based on biological composting, regular applications of plant and soil probiotic, and the use of intercropping, cover crops and living mulch.

Tropical soils hold nutrients in the plant bio-mass, the soil microbes and the soil’s parent material (i.e. volcanic minerals), this is one of many reasons it is so important to have cover crops in the tropics, bare ground loses nutrients quickly and promotes pests and disease.
The Transition Process:

Typically, we strip or burn the forest to make our clear patch for growing a food crop of whatever sort. In doing so we create a significant disturbance to the former balance of soil process. Soil process are profoundly determined by the interactions between the soil’s parent material, plants and the associated microbiology. The microbiology is the delivery system that feeds the plants from soil nutrients and feeds the soil from the photosynthetic nutrients created by plants. It is a highly productive system of natural balance, ‘no one needs to fertilize the rainforest’.

Commonly, trees were cleared but not always chopped down – many times they were simply ringbarked and left to die. In the case of ringbarking, of course the stumps were not removed and:

  • The nutrient from the dying tree was left to feed important soil microbes.
  • The soil was much less disturbed.
  • The leaf litter from the dying tree also helped to protect the soil during the interim period of moving from a stable forest environment, through the disturbance cycle, and into a pasture or crop production system.- this enabled early agriculturists to produce high value production at low expense.

During most clearing processes, trees were chopped down, used, sold or just burned. This often included the clearing of the stumps – causing the further loss of organic matter in the soil, loss of food for the microbes, loss of soil structure and ultimately reduced long–term profitable yield.

When trees were cleared, the animals that were associated with those trees either moved on or died, and the newly cleared area lost that source of nutrients provided by the animals. This included the essential microorganisms that lived in the soils and on the plant surfaces. Habitat means diversity in the system, and with diversity we are more able to achieve natural balance. With natural balance, we are more able to achieve profitable production (e.g. loss of birds that control insects, loss of fungal mass and associated soil structure).
Disturbance Management:

When we are moving from a natural forest system to a controlled agricultural system, these are some of the important elements to consider:

  • Ringbarking the trees and leaving them standing. Consider this option, as it allows through the light required to establish a pasture system with minimum soil disturbance and maximum preservation of the original forests ‘natural capital’. Planting an orchard amongst a ringbark system of course has the hazards of falling debris and the orchard would be planted in a pattern of design rather than just straight rows.
  • The Permaculture ‘food forest’. This is an example of one type of productive system that may be appropriate. Dropping the ringbarked trees after they have lost their leaves and before planting the orchard has multiple benefits.
  • Dropping trees and leaving the stumps cut off at ground level. This preserves an enormous amount of organic matter in the ground and reduces soil disturbance. Organic matter becomes food for the microbes, maintains soil integrity, soil structure and reduces the work required to clear the area.
  • Chipping. Should you drop trees that have no reasonable commercial value, then in this day of sophisticated equipment, we can chip the trees and leave the chipping on the ground as mulch to dramatically help hold the nutrient load in the soil for the benefit of our next crop. Note: It is of importance to understand that the sooner we chip the debris, the more nutrient we will save, because we have not lost the nutrients from the bark and leaves to oxidation.
  • Composting material. Should we remove the forest or regrowth and set this aside as chipped material, it is an important input for composting, returning this microbial/mineral rich organic material to our cropping system. This composted biomass of former forest material will save immensely on required input for the next cycle of your preferred cropping, especially if we include the leaves and bark.

In tropical soils, we hold nutrients in the soil microbes and plant biomass. Nutrient cycling happens rapidly and this constant nutrient conversation from dying material of the forest floor into new growth is rapid and constant.

If we are to mimic this constant flow of organic matter in our tropical productive systems, then, with the aid of modern composting equipment, we can supply that flow along with a high diversity of essential plant and soil microbes by applying compost. Nutrient cycling of the organic matter (formerly cycled by the forest floor debris) is now kept intact by our constant supply of compost.

Remember that proper compost is the precursor of creating humus, rich in both organic matter and in beneficial plant and soil microbes. Now we are well on our way to establishing a system that achieves the requirements of sustainable agriculture for the tropics. Here we define sustainable agriculture as ‘our ability to build soil fertility, whilst improving production and reducing labour and inputs’.

Properly made compost is weed free, helping reduce the need for the immense task of weed management in the tropics. Unfortunately, we have become accustomed to the use of herbicides which arrest natural succession, nutrient cycling, and the ability for soils to maintain soil structure, making us intensely dependent on imported inputs.

Nutrient Cycling is the process of transforming non-living material into living biomass and is controlled both by the interactions of plants and their associated microbes and by microbes and their associated soil minerals. Nutrient cycling is essential for accelerating and maintaining plant production and hence profitability.


Plant and Soil Probiotic (Specialist ‘Compost Tea’):

Whenever we harvest crops, we disturb soils, and whenever we disturb soils we are at risk of losing important microbes and nutrients. Using a compost-based system in the tropics reduces the need for disturbance, or even tilling when harvesting one crop and planting another.

Whenever we replace one crop with another, different microbes make specific relationships with that crop that are different from the associations with the former crop. If we are to maintain nutrient cycling for maximum benefit and preserve a healthy relationship between productive plants and microorganisms, then we need to maintain the presence of a vast range of microbes to keep these relationships intact. Regular application of compost teas means that we become less susceptible to disease and maintain all the benefit of nutrient cycling. Otherwise we are again dependent on expensive inputs, increasing workload, inviting more pests, disease and continually struggling to achieve profitable production.

Here we refer to ‘compost tea’ as a plant and soil probiotic because of the gross misunderstanding of what compost tea is. Compost tea is rich in aerobic plant and soil microbes whose life depends on structured soils, soils that hold air pockets and are able to accept and hold water and build all important humus. Remember, we can’t make good compost tea if we don’t make good compost, and we can’t mimic Natural Process without the consistent supply of good compost.

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