Chapter 8. Other Organic Fertilizers


Green manures offer succulent growth which decomposes easily in the soil, releasing nutrients. Green manures also smother weeds, keep nutrients from leaching, and protect the soil over winter. Permanent cover crops conserve nutrients, support a high biological activity, and produce a superior soil structure. The fertilizer value of green manures is difficult to predict because of the lack of knowledge of their abiliity to scrounge for nutrients, except in rare cases or with weeds.

The nutrient content of hay and straw can be estimated from tables 11. Nutrient Content Of Hay and 12. Nutrient Content Of Straw , and of weeds, from table 15. Nutrient Value Of Assorted Weeds .

Wood ashes are a source of potassium, phosphorus and trace elements in addition to a limited liming capability.

Purchasers of commercial organic products should obtain data on their total organic and nitrogen content.

Soil activators may have value in an unusual situation, but some may further deplete a barren soil.

Cover Crops

Characteristics of Cover Crops

A cover crop is any plant which improves the soil on which it grows. The actual benefits depend upon whether the plant is a perennial or an annual. Cover crops in pastures, hayfields and orchards consist of grass or mixtures of grasses and legumes. Some legumes, principally clover, are grown for hay for one or two years in rotation with grains and row crops Hedgerows are effective cover crops, since they protect the soil from erosion. The greatest benefit of perennials to the soil they are grown on is from the activity of their roots.

Annual cover crops are grown before or after a vegetable crop or in preparation for a field crop such as hay. They are usually grown as green manures: turned into the soil while still green, normally just before they flower. The greatest value of annual green manures is usually in the topgrowth, but the roots are also beneficial.

Green manures are chosen for their fast, vigorous growth and high production of green, succulent topgrowth. The topgrowth decomposes quickly in the soil and produce a flush of biological activity and a quick release of nutrients, some of which the roots may have accumulated from the subsoil.

Green manures rarely increase the organic content of the soil. The more likely affect is to cause the soil organic content to drop. This is owing to the tillage associated with turning the green crop under and with seed bed preparation. Green manuring establishes a dynamic soil system, where some of the humus is replaced by succulent residues, leading to a temporarily high biological activity and rapid availability of nutrients.

Green manuring is not practiced in dry climates where water is chronically a limiting element to plant growth. Considerable soil water is lost in transpiration from the green manure plant, lowering the water table, and following crops are likely to suffer.

Even in humid areas, however, green manuring is not popular, because it requires that land be left idle for a period of time, an unpopular practice for most market gardens on expensive land. Consequently, except when legumes are used to fix nitrogen, green manures are not usually grown as a fertilizer. Nonlegumes may be grown as smother crops to control weeds, and they are commonly used as a winter cover and catch crop, to protect the soil from erosion and to conserve nutrients otherwise lost by leaching.

Traditional choices for green manures are buckwheat, small grains such as rye and oats, and annual grasses such as sudan grass and annual ryegrass. Other varieties are possible, however; rape, for example, has been used as a green manure occasionally and is an excellent choice, so long as it is not followed by another brassica (which could encourage clubroot disease).

Legumes are also possible; Austrian winter peas are commonly planted in the fall for overwinter growth where winter temperatures are not severe; but they or other field peas may be grown also in cold climates in the spring or late summer. Soybeans were once popular, and sometimes hairy vetch is grown as a winter cover crop.

Most legumes, however, with the possible exception of Austrian peas, are expensive for use as a green manure. Also, with the exception of soybeans, legumes do not have a good leaf coverage, and they are poor smother crops; in fact they encourage weeds, owing to the nitrogen sloughed off their roots. They are best grown in combination with a nonlegume.

Planting a living mulch, or undersowing (or overseeding) a cash crop with a cover crop, is a variation of green manuring. Undersowings have attractive features: they provide a ground cover during the growing season and after harvest of the cash crop, and they compete with weeds. Legumes will fix nitrogen.

The disadvantage of a living mulch is that it also competes with the cash crop for moisture, nutrients and light. To minimize competition it should be sown late enough after seeding the main crop in order to minimize competition with the main crop but early enough to compete with weeds1.

Permanent cover crops do not have the vigor of green manures. Nevertheless, biological activity can be high; in fact average biological activity is higher under a good grassland than any other plant system. In contrast to the dynamic transience associated with a green manure, biological activity under grass is stimulated by the attainment of a stable, undisturbed state with a good environment (near-neutral pH, good drainage).

The sod not only feeds soil organisms but also protects the soil from the direct sun and rain for the entire year. Furthermore, the slow release of nutrients from dead roots and topgrowth minimizes leaching losses. Nutrients are strongly conserved and pass back and forth among the plants and soil organisms. The combination of a high biological activity and the extensive root system of grass leads to a superior soil structure and a slow but steady increase in soil humus.

When a sod is plowed under, it has a effect similar to a green manure: a rapid breakdown occurs with a sudden release of nutrients stored over a long period of time. A cultivated crop following a sod is usually successful.

Fertilizing Value of Cover Crops

The fertilizing value of a mulch obtained from a cover crop can be estimated from tables 11. Nutrient Content Of Hay and 12. Nutrient Content Of Straw . An example is in chapter 6. Unprocessed Residues - Manure Application Rates .

The fertilizing value of a crop turned into the soil in which it was grown, however, is difficult to determine. One can't tell simply by measuring the nutrient content of the plant. There is no way to distinguish between nutrients scavenged from the subsoil in a normally unavailable state and those already available.

Not much is known about scavengers; statements can be found indicating that one crop or another is good at picking up a particular trace element, but such reports are difficult to verify. The best commonly accepted examples of a green manure collecting unavailable nutrients are buckwheat and sweetclover, which have reputations for accumulating phosphorus.

One way of inferring the value of deep-rooted cover crops is to look at weeds, or at least those weeds which grow on depleted soil. If the soil is infertile and the weeds deep-rooted, a reasonable conclusion is that they are getting their nutrients from the subsoil. Table 15. Nutrient Value Of Assorted Weeds [67] shows the nutrient content of some weed varieties. The nutrients are listed in terms of lbs/ton in order to facilitate a comparison with other residues.

Woody plants have a reputation for picking up unavailable minerals. Apples, grapes and blueberries, for example, seem to grow better than one might expect with a meager supply of phosphorus.

Organic Byproducts

Table 16. Nutrient Content Of Processing Wastes is a summary of waste materials that have been recycled at one time or another [19], [63], [70], [71]. Depending upon their resistance to decomposition, they can be turned into the soil directly, composted or used as a mulch. Most are rarely found, except wood ashes, which are common enough to warrant additional remarks.

Wood Ashes

The principal nutrients in woods ashes are potash, usually 3 to 8%, phosphate, usually 1 to 2-1/2%, calcium, 20 to 25%, and magnesium, about 2%. Ashes also contain trace elements immobilized in the woody tissue. Although ashes are commonly recognized as a source of potassium and limestone, in addtion phosphorus, though low, is highly available. With these elements present, plus the trace elements, ashes are a good fertilizer for legumes. The liming value of wood ashes is about 2/3 lb of lime for each pound of ashes, as estimated in appendix C. Acid and Basic Nature of Fertilizers .

Ashes from a typical household are a practical fertilizer only for a small to moderate garden. A cord of wood may reduce to about 40 lbs of ashes2; this has an liming value equivalent to about 25 lbs of lime and has about 2 lbs of potash. Meeting a 100 lb potash demand for a one acre field with wood ashes, for example, would require ashes from 50 cords of wood.

On the other hand, the ash supply from a house which burns four cords of wood, used on a garden of 3000 sq ft, would add lime at the equivalent rate of about 3/4 ton/acre and potash at the rate of about 120 lbs/acre. Spread every year, such a quantity would meet the potassium requirement of a crop or at least come close, but it might also eventually raise the pH to an excessive level. A seven cord household with a 1000 sq ft garden would be in trouble very soon.

Wood ashes should be handled with care; they are caustic and may injure anyone working with them, and they may injure soil life. Composting ashes might be preferable to spreading them directly. They should be added to a manure compost, however, only after the first stage of breakdown, to avoid unnecessary loss of nitrogen.

Coal ashes are not the same as wood ashes. They have no nutritive value and may contain toxic substances. Coal ashes can improve a heavy soil if they are first screened to select only the finer particles and then soaked to leach out toxic elements.

Commercial Organic Products

Many organic residues are available commercially as dried and ground preparations. Table 17. Nutrient Content Of Commercial Organic Products [19], [63], [70], [71] lists the more common materials. Some will be discussed in more detail in chapter 10. Nitrogen .

Commercial compost produced by companies having access to large quantities of waste products is a good alternative for those who are not able to make their own. Unfortunately, a fair appraisal of commercial compost is difficult. Current labeling laws require specification of water-soluble nitrogen and potassium and citrate-soluble phosphorus. In practice, however, knowledge of the organic content and the total nutrient content, or at least the total nitrogen content, is more useful. Anyone using large amounts of purchased compost should write to the manufacturer for these specifications.

In particular the total nitrogen content gives the best estimate of the available nitrogen. About half of the total nitrogen should become available during the first year of application.

Knowledge of the total phosphorus and potassium content of compost is useful, though not as valuable as nitrogen. Unlike nitrogen, phosphorus and potassium could exist in commercially available compost partly as rock powders; if so, their availability would be impossible to estimate.

Uncomposted commercial blends of organic materials and rock powders are also for sale. Like compost, their value is specified only in terms of the readily available NPK content. The organic and total nutrient analysis would be useful, though probably not as valuable as it would be with compost. The actual composition may vary, depending upon the manufacturer's inventory. Also nitrogen availability would be difficult to estimate. The nitrogen from most residues is quickly available, except for leather meal, which is a major ingredient of some blends.

When using any material containing leather waste, one should enquire about the presence of chromium.

Soil Activators

A completely different category of materials are the various proprietary products sold as soil activators. They are intended to boost biological activity or to use enzymes to increase the availability of soil nutrients. Nitrogen is the usual target.

Whether these activators are useful depends on the soil to which they are applied. If the soil is already fertile, activators are unnecessary. Where they may be most useful is on depleted soils. Some people feel that these products are useless and a waste of money. It seems reasonable, however, to believe that circumstances exist in which they do work, that is, they result in the release of available nitrogen.

A greater concern, however, is the damage they will do if they are successful in a poor soil. The nitrogen that they manage to extract will come from oxidizing the little quantity of organic matter that still is in the soil. They will accelerate the loss of organic matter and leave the soil worse off than before.

Activators which do not exploit the soil organic matter but help plants obtain minerals may be better - if they work. One example is humates, which appear to stimulate biological activity in the root zone. Such biological activity includes mycorrhizae, which are able to increase the availability of minerals, particularly phosphorus. Whether or not humates are effective is still in dispute. If they are, and if the increased biological activity feeds off of carbohydrates produced by the plant roots rather than the soil humus, these activators may have merit. Compost also contains humates and is preferable because it has many additional beneficial elements.

Before choosing any activators, it would be worthwhile to compare them with compost in a field experiment. A conclusion should include an analysis of costs and benefits.

1 See articles in New Farm magazine for current information and practices.    [return to text]

2 According to W. Erhardt, University of Maine, who obtained his information from two sources. One states that the ash content may vary from 50-60 lbs/cord, and the other that a variation of 8-40 lbs/cord is likely.    [return to text]

© 2013 Robert Parnes

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