Tips for Working with Tropical Soils

Tips for Working with Tropical Soils

Lydia is one of our 2016 Apprentices in Sustainable Living. She has studied Horticulture and has a Bachelor of Environments (Landscape Architecture) from the University of Melbourne in Australia. She is super keen on plants and the fundamental relationship between plants and soil. Healthy soil is the key to healthy plants. This post is about soil in the tropics, including some of the challenges that exist and how these challenges are managed at Rancho Mastatal.  

Sacha Inchi before a heavy mulch application.

Sacha Inchi before a heavy mulch application.

Soil, the ever fascinating topic to any garden guru or green thumb. Here at the Ranch it's all about topsoil, which is basically non-existent. The little soil that is present has a high clay content making it particularly susceptible to compaction. It is also highly acidic. The combination of the two means that without any additives it would be challenging to grow anything at all. So we've devised a number of methods to build up the topsoil, break up the clay and raise the pH throughout our orchard and garden spaces. These methods can be used anywhere in the world to improve your soil.

The same plants as above after a healthy mulch cover.

The same plants as above after a healthy mulch cover.

First and foremost, topsoil! How do we do it? The majority of nutrients in the jungle ecosystem are stored in plants. When nutrients become available plants are quick to soak them up because there is stiff competition when living in such close proximity to your neighbors. This means that plant biomass is an incredibly rich and abundant source of nutrients. Plant biomass refers to all plant material, living and dead including leaves, trunks and branches. The main way we utilize this biomass is what Scott refers to as "doughnut" mulching, basically forming circular doughnuts of plant material around the base of our fruit and nut trees. This mimics nature by following the 'top-down' approach that allows microorganisms to move the material down. It is important to note that mulch should never touch the trunk of your trees because this will cause rot. A 5 inch radius between trunk and mulch is a good aim.

Another method used by Rachel in zone 1 to improve soil quality is to bury pieces of wood under the soil in our garden beds. The wood acts like a sponge, retaining water throughout the dry season and, reducing compactness in the wet season by aerating the soil. It also provides habitat for microorganisms, crucial for plant health and growth.  Our garden beds are mulched regularly with dead leaves collected from the forest floor to retain moisture in the soil, reduce weed growth and add nutrients. 

Now, onto soil pH, the next big topic around soil in the tropics. There are 6 cooking methods used at the Ranch, 3 of which rely on wood, the rocket stove, the biochar stove and the pizza oven. With 3 meals per day, 6 days a week being pumped out of our kitchen, you can imagine how much charcoal and ash we produce. Instead of simply disposing of this alkaline-rich bi-product we add it to our soil. 

Charcoal is a carbon-rich soil additive, that adds alkalinity to the soil, raising the pH. It has a high surface area on a microscopic level which means it is capable of storing water and nutrients in the soil, making it a valuable additive in both the dry season when water is required and the wet season when nutrients are required. The addition of charcoal also aerates the soil and breaks up the clay. It is essential to note that when adding charcoal to soil it must be soaked in a nitrogen solution first, such as compost/ worm tea. This will ensure it doesn't lock up precious nutrients from the soil. Ash is similar to charcoal but much finer. It also adds alkalinity to the soil, raising the pH. Limestone is another soil additive used to raise the pH and up the alkalinity. We spread lime around the base of our fruit and nut trees, vines and shrubs. 

Lime applied to a raised garden bed.

Lime applied to a raised garden bed.

Nitrogen is one of the most important soil nutrients and in order to produce good crops, nitrogen is essential. What better way to introduce nitrogen into the soil than with nitrogen fixing plants. These plants produce "cake & cookies" i.e. sugars in the soil that attract nitrogen fixing bacteria. These bacteria are capable of using these sugars to convert nitrogen gas in the soil into a plant soluble form, accessible to the surrounding plants. Believe it or not, plants control this process depending on how much nitrogen they feel they need.T he more sugars the plants produce, the more sugars the bacteria consume, converting this sugar into nitrogen for the plants. It's an incredible natural process! The nitrogen fixers most extensively used here are Acacia mangium, Shizolobium paryhybum, and Madero Negro, all of which are trees. We also use peanut grass, a spreading ground cover. Most plants in the Leguminosae family can be used as nitrogen fixing plants. 

Making Biochar with a TLUD System.

Making Biochar with a TLUD System.

Potting soil is another essential part of our plant operation here. The recipes used for potting soil change depending on what's available and required. One of the recipes Rachel has devised is 2 parts forest soil plus 2 parts worm farm compost plus 1 part sifted builders sand. We collect the forest soil from the forest floor where a build up of plant material is prevalent e.g. a puddle, low point or valley. This is where nutrients gather because soil and small plant material is deposited here. The worm farm compost is a bi-product from our bio-digester system. This waste treatment system converts cow manure into methane gas, while leaving the fibrous material behind for the worm farm. The key to a great potting soil is to balance soil pH and ensue there is a rich source of matured compost where microorganisms thrive. 

So there you have it, the challenges and of course the fun of soil in the tropics! Creating and enhancing topsoil, balancing soil pH, in this case raising it, balancing the clay content of your soil and making nutrient rich potting soil. I hope you can take some of these tips away and enrich your soil wherever you are. Here's to happy soil and abundant plants.

Lydia Blocksidge

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