Hesperian Health Guides
Improving Soil
HealthWiki > A Community Guide to Environmental Health > Chapter 15: Sustainable Farming > Improving Soil
Farmers know that healthy soil is necessary for good crops. Many farmers enrich soil with natural fertilizers, such as animal manure, green manure, and compost. Natural fertilizers are healthier for soil, plants, water, air, and people than chemical fertilizers. They add all the nutrients plants need at little or no cost.
Contents
Know your soil
Soil is a mix of sand, silt, clay, and organic matter (for example, insects, bacteria, green leaves, rotting plants, and manure). The amounts of each of these things, and the way you work the land, affect the soil texture (how coarse or fine it is), fertility (how rich it is for growing crops), and soil structure (how the soil holds together). A soil that has good texture, structure, and fertility allows air, water, nutrients, and plant roots to move through it. This improves the soil’s ability to grow crops and resist erosion.
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| Sustainable farmers not only grow crops — they grow fertile soil that has all the nutrients plants need. |
In addition, some soils are alkaline (also called “basic” or “sweet”) while others are acidic (also called “sour”). You can learn the “pH” of your soil (how sour or sweet it is) by having it tested or simply by tasting it to see if it is sweet or sour. Most plants grow best in soils that are neither too sweet or too sour. Adding specific nutrients can make soils sweeter or more sour. Adding organic matter tends to improve all soils.
Using heavy equipment to plow, till, turn over, or dig soil can make it become compacted (pressed down so tightly that no air or space remains). It is difficult for water or plant roots to get into compacted soil. It is also difficult for plants to get the nutrients they need from soil that is compacted.
To prevent compacting soil, clear and turn over soil when it is not too wet or too dry, but moist like a wrung-out cloth. Many farmers turn their soil as little as possible, add animal manure and crop wastes, and use methods such as planting pits or green manures to make the soil loose for planting.
Chemical fertilizers may help now, but can harm later
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| Chemical fertilizer |
Chemical fertilizers are costly to both the farmer and the farm because they damage soil, pollute water, and create the need for more chemicals. If you look at a bag of fertilizer from the store, it will have the letters N-P-K. These letters stand for the main nutrients that plants need (N is Nitrogen, P is Phosphorous, and K is Potash, or Potassium). Chemical fertilizers have these chemicals in concentrated (very strong) amounts. When these concentrated nutrients are washed from fields into groundwater and waterways, they can make the water unhealthy for drinking, washing, and bathing.
The biggest problem for growing crops with chemical fertilizers is that farmers who use them often stop adding organic matter, such as animal manure, to the soil. This very quickly causes soils to lose nutrients and become compacted, leading to pest problems, poor harvests, water loss, and more dependence on chemical fertilizer. If you use chemical fertilizers, it is important to add natural fertilizers along with them.
Learning about soil
Purpose: This activity helps show how different farming practices affect the soil
Time: 3 hours
Materials: digging tools, 3 boards or pieces of cardboard, water, paper, and a pencil or marker
- Choose 3 parcels of farmland that have been used in different ways. For example, choose a field of maize or dry farmed rice, an orchard or home garden, and a plot that has been used for pasture for many years. The plots should be within easy walking distance from each other.
- With a group of farmers, walk through each of the areas. Cross back and forth, looking at everything that may have affected the soil. What signs show how the land has been used? Are there signs of erosion (for example, gullies, bare or rocky spots of ground, richer soil at the bottom of hills than at the top)? Do the plants look healthy?
- Talk to the person who farms each area to find out what practices they have used over the past 5 to 10 ten years. Do the group’s observations match what you learn from talking to the farmers?
- Dig a small pit about 50 cm deep in each parcel. Cut 1 wall of the pit so that it is straight down and flat. Using a flat shovel or a long machete cut a slice about 3 cm thick from the flat side of the pit. Lay this slice of soil gently on a board or flat surface. Label the soil sample to identify which parcel it came from.
- When you have taken soil samples from all 3 areas, bring them to a meeting place where the group can examine them. What differences are there between the different soil samples? Look closely for differences in color, texture, structure, smell, and the presence or absence of worms and insects. Perhaps taste a small bit of each soil to compare the pH. Is it sweet or sour? Have different people take a little soil in their hands from different samples. Work in a small amount of water to each and say if it feels sticky, rough, smooth, or falls apart.
- Discuss which of these differences may have been caused naturally by wind and weather, and which may have been caused by the way the land was used.
Using knowledge from the group, from this book, or from other sources, discuss ways to protect and improve the soil in the areas that will be used for farming. These ways may include adding natural fertilizers, protecting the soil from erosion, using sustainable grazing practices for livestock, and trying other farming practices.
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| The small balls (nodules) on the roots of legume plants put nitrogen into the soil. |
Green manures and cover crops
Green manures are plants that help fertilize the soil. These same plants work as cover crops to choke out weeds. Since many plants do both these jobs, they are called by both names: green manures and cover crops.
Many green manures are from the "legume" family (plants with seed pods, such as peas, beans, and tamarind trees). Plants in the legume family add nitrogen to the soil. If you pull up a bean plant, or look at some tree roots, you will often see small balls formed on the roots. These little balls collect nitrogen from the air and put it into the soil. This makes the soil more fertile.
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| Fava bean Vicia faba |
Alfalfa Medicago sativa |
Runner bean Phaseolus coccineus |
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| Sorghum Sorghum |
Lablab bean Dolichos lablab |
Velvet bean Mucuna pruriens |
Green manures have many benefits:
- They cover the soil, protecting it from erosion and helping it hold water.
- They add organic matter to the soil, making it more fertile.
- After using green manures for several years, the soil becomes easier to work.
- There are no costs for labor or transport because green manures grow right in the field where they are used.
- Planted with other crops, they control weeds and insect pests.
Green manures have other uses besides improving soil. Some provide food, such as oats, amaranth, rye, and beans. Others provide fodder for animals, such as alfalfa and clover. Plants such as Sudan grass and others in the mustard family prevent crop diseases. Trees used as green manures can provide firewood.
3 common ways to use green manures
- Grow them together with main crops such as maize, millet, and cassava.
- Plant them when the land is going to be left to rest (fallow). A 1-year fallow with green manure will improve soil and kill weeds just as well as a 5-year fallow with no green manure.
- Grow them during the dry season, after the main crop is harvested.
The best cover crop is a mix of plants. A grain that grows fast and tall will add organic matter to the soil, while a bean crop will add nitrogen and will cover the ground. Talk to other farmers in your region to learn what works best on your soils.
Mulch
It is best to keep soil covered, even during the growing season. Mulch is anything used to cover the soil. Mulch helps hold water, control weeds, and prevent erosion. Plant wastes, such as maize stalks, bean vines, or grasses make the best mulch, because they can be simply left to rot in the field, and they add organic matter to the soil. Weeds can be used in the same way, but they must be cut before they make seed to prevent them from growing back.
Mulch should not be more than 10 cm thick. A very thick mulch can hold too much moisture and cause plant diseases.
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Do not let mulch touch the stems of plants. It can cause plants to rot.
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| Straw and grass cuttings make good mulch because they break down slowly. |
Animal manure
Animal manures provide all the nutrients plants need, and over time improve soil texture, soil structure, and soil fertility. Chemical fertilizers, on the other hand, give crops only 2 or 3 nutrients and do not improve the soil.
Some care must be taken with manure. Using too much manure will cause too many nutrients to build up in the soil and can pollute waterways. Fresh manure also carries germs that can cause illness. Do not put fresh manure near drainage ditches or waterways. Always wash your hands and your clothing well after handling manure.
Fertilizing with human waste
Human urine can be turned into fertilizer, and human feces when properly treated can add organic matter to the soil. But human waste carries harmful germs and causes illness if it is not properly handled. (To learn how to safely use human waste to improve crop yields, see Ecological Toilets in Chapter 7.)
Compost
Compost is a natural fertilizer made of food scraps, crop waste, weeds, and animal manure. Adding compost to the soil is a way of adding crop nutrients back to the earth. It would take a lot of work to make enough compost for a large field, so compost is most often used on smaller plots. (See “Making compost.”)
Compost can be used in many ways:
- Add a shovel full of compost in the bottom of planting holes before planting fruit trees.
- Mix a handful of compost with soil in planting holes when you plant seeds.
- Spread a layer of compost on top of your soil before turningnbsp;it.
- While plants are growing, make a circle of compost around the plant stem. For a tree, make the circle where the edge of the tree’s shade falls in the middle of the day. Cover it with a little soil. It will slowly feed the plant as water carries nutrients to the roots.
Compost tea
Compost can be used to make a liquid to fertilize plants and help control pests. Wrap some compost in a piece of cloth and tie it up. Put the cloth in a bucket of water for 7 to 14 days. When the water turns brown, take the cloth sack out. Spread the leftover compost in your field. Spray or sprinkle the compost tea on the leaves of your plants. Be sure to wash your hands after working with compost tea.
Other ways to add nutrients to soil
Other materials can be added to change soil pH and to add nutrients to the soil. Limestone, wood ash, and ground animal bones and seashells make soil less acid. The ground up animal bones also adds phosphorus and the wood ash adds potassium. Dried leaves and pine needles make soils more acid. Sugar cane that has rotted for at least a year and coffee pulp that is ground and dried add nutrients to soil, turning crop waste into fertilizer.
Improving soil helps control weeds
All of the methods of improving soil with organic matter, such as green manures, compost, and mulch, also help control weeds. When the soil is healthy, small amounts of weeds do not harm crop yields.
Weeds can also be controlled by planting crops close together so there is no room for weeds to grow, and by allowing animals to eat the weeds. Also, crops that are native to the area tend to be harmed less by local weeds. Over many years, locally bred crops adapt to weather, weeds, and pests, and do well where other crops or other varieties of the same crop may not.