Sustainable Agriculture Extension Manual

Using organic matter

Maintaining soil fertility is one of the most important things farmers can do. Here are some questions about the soil and the types of organic fertilizer they use. The answers will help determine how best they can maintain and increase the soil fertility.

What do you know about the soil?

You should know the characteristics of your soil and its problems. Is it short of nitrogen? Is it low in phosphorus? Does it have enough organic material?

If possible, get a soil test done by a laboratory or extension agent. The test will tell you if the soil is lacking essential nutrients and what you can do about it.

You can also use the following simple rules of thumb:

• If the crops have yellow leaves, the soil may be short of nitrogen. You may need to add nitrogen in the form of organic material or artificial fertilizer.
• If the crop leaves are purple, the soil may be low in phosphorus. You should add phosphorus-rich organic material (such as tithonia leaves and stems), and preferably artificial fertilizer that contains phosphorus (such as di-ammonium phosphate, DAP).

Note that the crop leaves may become yellow or purple because of waterlogging, competition with weeds, diseases, or other reasons. So these rules of thumb are not a certain guide. If in doubt, check with an extension agent.

What types of organic material do you have?

Farmers have many different sources of organic material they can use to keep their soil fertile. Some comes from the crops they grow. Some comes in the form of animal manure. And some is in full view in hedges, roadsides and field edges: weeds and wild plants that farmers often do not recognize as a valuable source of organic material.

Look at the types of organic materials you use, or could use. The table on the previous page shows the types of organic material that may be available, as well as some of the constraints or problems you may face when using them.

Plants How best to use plant organic material depends on the types of substances it contains. Plants contain three main substances that may make them good organic fertilizer—or not-so-good. These substances are nitrogen, lignin and phenols. Nitrogen. All plants need nitrogen to grow. They get it from the soil, and they store it in their leaves, stems and roots. Some types of plants are good at getting nitrogen either from the soil or from the air, and others are not so good. Tithonia, for example, is very good at taking up nitrogen from the soil, while maize is less good. Some plants, called legumes, even fix their own nitrogen from the air. Legumes (beans, chickpeas, sesbania, leucaena and many others) are very valuable organic material or intercrops as they actually increase the amount of nitrogen in the soil as well as storing it in their stems and leaves (see the sections on Green manuring and Inoculating legume seeds). In general, you can tell if a plant contains a lot of nitrogen from the colour of its leaves. If it has dark green leaves, it probably contains a lot of nitrogen (so makes good organic fertilizer). If the leaves are yellowish, it contains little nitrogen (so is less useful) (Picture 1).

Dark Green Leaves High in nitrogen Good organic ferilizer Yellowish leaves Low in nitrogen Poor organic fertilizer

Can tear leaves easily Low in lignin Good organic fertlizer

Lignin. When they die, some plants rot quickly and release the nutrients they contain into the soil within only a few days. Other plants decompose more slowly, so their nutrients are released only gradually. In general, plants that rot quickly make better organic fertilizer, as the crop can use their nutrients sooner.

Plants that are woody contain a substance called lignin. In fact, all plants contain lignin, but in different amounts. Plants that contain a lot of lignin rot more slowly, so do not make very good organic fertilizer. Older plants tend to contain more lignin than younger ones.

You can tell if a leaf contains a lot of lignin by tearing it with your fingers. If it tears easily, it does not have very much lignin (making it good for use as fertilizer). If it is hard to tear, it has a lot of lignin and will not make good fertilizer (Picture 2).

Phenols. Phenols are substances in the plant that also make it rot slowly. A plant with a lot of phenols will make low-quality organic fertilizer. You can tell if a plant has a lot of phenols by tasting it: if it is astringent (it makes your tongue curl up), it probably has a lot of phenols and will make poor organic fertilizer (Picture 3).

Different parts of plants may contain different amounts of these substances. For example, the stems may contain a lot of lignin, but the leaves may have very little (so it may be best to use only the leaves as fertilizer).

The amount of these substances also depends on where the plants grow. A plant growing in a warm, rainy place on good soils may contain different amounts of the substances from the same type of plant growing in a dry, poor soil.

Other things can also influence whether the plant will make good organic fertilizer. For example, if the leaves are waxy, they will decompose slowly, so make poor fertilizer (Picture 4).

All this makes it difficult to say for certain what the best way to use a particular type of organic material will be. It is best to try out various things to find the one that best suits your farm.

Animal manure

The quality of manure depends on what the animals have eaten. If they have been fed with poor-quality forage or grazed on poor soils, their manure will be of poor quality. If they have been fed good-quality feed, the manure will be rich in nutrients (Picture 5).

Manure needs to mature for several weeks or months before it can be used as fertilizer. But the amount of nitrogen it contains falls over time, because nitrogen escapes into the air or is washed away by rainwater. To prevent this, keep it in a covered pit or drum before spreading it on the field (Picture 6).

Manure with urine (for instance, if it is collected in a lined slurry-pit) contains more nitrogen than manure without urine. But the nitrogen in urine is lost easily. Cover the slurry-pit to prevent this (see the sections on Zero-grazing and Urine–manure slurry as fertilizer).

Even if manure is of poor quality, it still makes useful fertilizer, so you should use it anyway on your soil.

The type of manure also influences its quality, because different species of animals eat different things. In general, manure from pigs and poultry is of better quality than manure from goats and cattle. You can enrich cattle manure by mixing it with manure from other types of animals.

 How should you use organic material?

Many farmers do not use the organic resources they have available wisely. Up to 40% of the nitrogen in manure is washed away by rain or disappears into the air before the manure is used as fertilizer. In western Kenya, only 10–20% of farmers use plants from hedges to help improve their soil. Farmers face many choices in deciding how to use organic material. They can:

• Choose among different plant types (trees, cover crops, crop residues, etc).
• Select organic material from different places (on-farm or off-farm).
• Pre-treat the material in different ways (apply it directly, make compost, etc).
• Mix different types of organic material, or use a combination of organic and artificial fertilizer.
• Spread it on the surface as mulch, or work it into the soil.
• Apply the organic material before ploughing or afterwards.
• Use the organic material to fertilize high-value crops such as vegetables, or on staple crops such as maize.

Once you have found out the quality of the organic material you want to use, you must decide how best to use it. Depending on the type of plant material or manure, it may be best to incorporate it into the soil, use it as compost, or leave it on the field as mulch.

It is best to use high-quality organic material directly as fertilizer on the soil. For other types of organic material, such as maize stalks, it is probably better to compost them first

(see the section on Composting), or mix them with inorganic fertilizer before incorporating them into the soil. Some types of organic material, such as stems and twigs do not make good compost because they contain too much woody lignin. It may be best to leave these on the soil to control erosion and help retain moisture. The table above has guidelines on how to use different types of organic matter.

If you have both high- and low-quality organic materials, mix them together before applying them on the field. Mix together plant and animal wastes to enrich poor-quality manures.

In general, it is better to use a mixture of organic and artificial fertilizers than either organic or artificial fertilizers by themselves. If you use high-quality organic material, you probably have enough nitrogen but not enough phosphorus—so fertilizers that contain phosphorus should be your priority, especially if you know your soil is low in phosphorus. Choose an artificial fertilizer that contains both nitrogen and phosphorus rather than one that has nitrogen only (see the table below).