HOW MUCH COMPOST SHOULD WE USE?
EHRENFRIED E. PFEIFFER
The question of the rate of application per acre is one that is frequently asked. More recently, another has come up: Can we use too much? Here is our advice. Make all the compost you can in your garden and on your farm, using all available wastes, and it will never be too much. In fact, in many cases it will not be enough. If you have to buy compost, manure or fertilizers made from organic wastes, buy with reason and with knowledge, not at random. The term compost as used in this article includes well-rotted manure – not fresh manure.
Maximum applications of 150 tons per acre have been seen, but on the average 5 to 20 tons per acre are more usual. Of dehydrated, skillfully manufactured and concentrated composts, as little as one ton per acre may be sufficient. The range is wide so common sense and some basic knowledge are necessary. When using inorganic fertilizers one is guided by their formulas and the apparent needs of the plants to be fertilized. While the chemical formula of the inorganic fertlilizer gives a lead as to its rate of application, the chemical formula of a compost does not entirely answer this question, although it can help in determining its usefulness. The customary NPK (nitrogen, phosphate, potash) analysis indicates only a small part of the total value of composts.
The chemical farmer and gardener knows the NPK and lime requirements of his crops. More recently he also considers (or should consider) magnesium and a long list of trace minerals. Only now is he beginning to appreciate the value of organic matter. He knows the behavior of heavy clay soils and light sandy soils. In practice he applies very little of the existing information about the availability of minerals. It is true that he buys available fertilizer elements. State laws prescribe the use of formulas on fertilizers with regard to soluble inorganic nitrogen such as sulphate or nitrate of ammonium, and organic nitrogen (urea, for instance); available phosphates (such as superphospate) as against total or unavailable phosphates (rock phosphates, for example); water soluble potash or available potash, to mention the big three (N P K). Lime is used in accordance with the degree of acidity of a field or its calcium deficiency.
Rarely, if ever, does the farmer or gardener ask: Do these fertilizers remain available in the soil? Are they efficiently used or wasted. In recent years, there has come to light a fact which can be of utmost importance; mineral plant food elements can be tied down in a soil. Lime can be applied by the ton year after year but the pasture or hayfield remains acid. It has been tied down and carried into the deeper layers of the soil where it cannot be reached by the plant roots. Actually such fields have been over-limed. In certain soils available phosphates can become unavailable almost immediately after application. Thus in alkaline soils they may become altogether unavailable right away, and in acid soils only a fraction of the phosphate may remain useful for the immediate crop, usually between 2 and 10 per cent, at best 20 per cent. The bulk of the phosphate fertilizer is tied down, has become unavailable. Lucky the farmer whose soil conditions are such that the phosphate fertilizer, due to microlife and cultivation, can be made available in successive years. Unlucky the farmer whose soil stores the phosphate or transports it to lower levels inaccessible to roots or even lets it wash out (chemical erosion vertically downward). Such soils remind one of the old story of a thirsty horse who is tied to a fountain, but with so short a halter that he cannot reach the water. The fountain can be ever running, the farmer will buy more and more; in fact, he finds that he needs more and more every year to hold his yield level. The term “water soluble potash”, prescribed by some State laws on the fertilizer bag, is especially deceptive. Water soluble means what it says. It is that part of potash which runs away with the water (rain or irrigation). It is also most easily absorbed by plants, too easily, in fact, and then leads to luxury consumption causing disturbances in the metabolism of the plants and the animals which eat them. Besides all this there are the problems of the inter-relationship of trace mineral availability or unavailability and the other fertilizer elements. One example is the adverse relationship between boron and lime.
Composts behave differently and cannot be judged entirely by their NPK formula. Nitrogen in compost is mostly organic nitrogen. Much less nitrates are present and very little ammonia should be present. Phosphates and potash are also present, but in forms which do not show up through the prescribed analytical procedures for determining availability or water solubility. At best, following customary analytical methods one can determine the total amount of nitrogen, phosphate and potash. But these methods do not take into account the fact that – due to the microlife in the soil and in the compost – these compounds are slowly gradually made available in soil when compost is applied.
Experience has shown that low nitrogen formulas (1 to 2%) in compost produced as good results as the application of high nitrogen fertilizer formulas (6-16%) with one important difference. Highly soluble fertilizer elements produce a rapid but not lasting effect while composts produce a slower but longer lasting effect. The effects of highly soluble and available fertilizer elements make the best showing during the first tree to six weeks after application. The elements in composts last for three to six months with a steady release, and if the soil is rich in organic matter, further beneficial effects can be seen in the second and even in the third year.
Now we come to the most important question. Not all composts are alike, either in formula or in behavior. Here the greatest mistakes have been made in the past by using composts indiscriminately under the far too general heading “compost” or “the organic method”. Of fertilizers there are many brands and formulas. There are also many different formulas of compost, maybe even many more than of fertilizers. A grading of composts is absolutely necessary if one wants to use them efficiently. But the intrinsic processes in a living compost and in soils should also be understood in order to arrive at maximum effects. The different types of possible composts must therefore be considered.
We will begin with the organic matter content of compost. It is incorrect, as one reads here and there, to say that a compost contains 100% organic matter. It is correct to say that it has been made from organic wastes such as plant (garden) refuse, leaves, garbage, sawdust, animal wastes, manure, etc. These materials are of organic origin because they derive from things which at one time were living organisms. If plastics are used they may be products of organic compounds. But most of these source materials also contain inorganic minerals. A living plant contains 2 to 10% inorganic compounds which upon combustion show up as ashes. Organic materials in strictly chemical terms are combustible and derive from complicated compounds of the chemistry of carbon, nitrogen, hydrogen, and oxygen, sometimes combined with other elements such as sulphur and phosphates as in proteins and their breakdown products; or carbon compounds like sugar, cellulose, hemicellulose; or combinations with calcium, magnesium and other mineral elements. A compost is a breakdown product of all these materials. Microorganisms have used part or all of these breakdown products and ‘digested’ them, building up new compounds which in turn again break down. It is of the utmost significance for the fertilizer value of composts to what degree these breakdowns, new syntheses, secondary breakdowns, have taken place. The final, end products of all breakdown processes are: carbon dioxide, simple nitrogen compounds such as N2, NO2 , NO3 , NH3 , and water; all of which can be lost in one way or another. Simple mineral compounds result such as calcium, magnesium, other nitrates, ammonia salts, carbonates as well as silicates and aluminates as they may happen to be present in the source materials. The organic matter content of a compost therefore can vary greatly.
Average farm and garden made composts contain a sizable amount of earth. The organic matter content is therefore low. In fact most of these composts range between 12 and 20% organic matter content, a percentage of 20 to 30 is rarely if ever found. Many piles, especially if they are earthy, old and well-rotted are nearer the 12% level and piles with less, even as low as 6 to 9%, have been frequently encountered in our analytical work. These are still called compost because of their origin and the way in which they have been treated, but actually they have become soil – a rich soil, though, rich in organic matter. Here a mineralization process has taken place and most of the carbon has vanished in the form of carbon dioxide. It is evident that such mineralized composts behave entirely differently when applied to soils than others with a high organic matter content. It is an illusion for the gardener to think that he can appreciably increase the organic matter in his soils by using low organic matter composts. While they still will be of benefit to the soil structure, improving soil particle agglomeration or aggregation, their main value is in their mineral content whatever it may be. Of these low organic matter composts a considerable amount has to be given per acre if any apparent results are to be obtained.
RULE I: Of earthy, well-rotted, low organic matter composts 10 to 20 tons per acre may be used, or ˝ to 1 lb. per square foot, or 500 to 1000 lbs. per 1000 square feet. No harm can be done at such rates. However, the term organic matter only tells us the amount of combustible compounds present but not the degree of breakdown. Crude compost may contain the same amount of organic matter as a well-digested material such as humus. The effect of a raw compost upon soil is quite different from that of a humified compost. It is the skillful and experienced compost manufacturer’s aim to produce a compost with a high organic matter content in a well broken-down state (humified), but not in an extreme state of mineralization. So-called stabilized products of simply broken-down crude organic matter are again entirely different products, even though their formulas may give the same NPK percentage as others. Such products hardly meet the definition compost. Higher rates of application than those given in Rule I are not desirable for many reasons, one of these being the increased packing and hardening of the soil due to hauling so many truck loads over the field, not to speak of the cost of the spreading operation.
A word must now be said about the moisture content. A homemade garden or farm compost is usually wet and in this state has a moisture content of about 50%; or it may be dry to the touch, and then the moisture will run from 20 to 30%. If it is dusty, it will probably contain less than 15% moisture. A yardstick to help in estimating moisture content and hints on how to acquire it were outlined in an article in BIO-DYNAMICS some time ago. (1) It should be realized that more of a material containing half of its weight in water must be used than of another containing only a quarter of its weight in water. The above given rate of 10 tons/acre applies to fairly dry but not dusty composts, and the rate of 20 tons/acre applies to wet or rather moist composts. A dryer compost contains a higher percentage of minerals than a wet one, one might say that it is a ‘stronger solution’. Analyses of composts should, therefore, not only give the organic matter and mineral content, but also the moisture content. For example, one can calculate the absolute content on a dry basis, but this is not the percentage that is applied in practice.
As far as crop growth and yield are concerned the nitrogen content is certainly decisive. We should know the nitrogen content. However, reference must again be made to the moisture content. We will fare much better if we know not only the percentage of nitrogen but also how many pounds per acre of nitrogen we are giving at the given rate of application. Most homemade garden and farm composts, manure from horses, cattle, and hogs, have a total nitrogen content of 0.5 to 0.7% at a moisture of 50% and more. Cattle manures have a moisture content of 80%. Poultry manure is the exception and can contain as much as 1.5% total nitrogen at high moisture levels. On a dry basis, the total nitrogen content of all of these will be higher. Dry compost will run from 1 to 2%; dry cattle (steer) manure may have as much as 2%, poultry manure 2.5 to 3% nitrogen.
Compost or manure of 1% nitrogen contains 20 lbs. of nitrogen per ton no matter what the moisture content is. If one ton of a 1% nitrogen compost is applied to an acre you are applying 20 lbs. of nitrogen. If ten tons are spread you will be giving 200 lbs. per acre. 0.5% nitrogen in a manure or home garden compost means that, at the rate of 10 tons/acre, 100 lbs. of nitrogen are applied, or at the rate of 20 tons/acre 200 lbs. of nitrogen are applied.
Two hundred pounds of nitrogen is the upper limit for most crops. Many corps require less, for instance 60, 80, or 120 lbs. It is evident that much less of a high nitrogen, low moisture compost need be applied per acre than the above-mentioned figure of 10 tons. In fact we have seen good results with the B.D. Starter produced compost when only 1 ton/acre was used. This compost was, of course, dry.
An insufficiently rotted, rather raw compost of low nitrogen content should not be used. Its further decomposition in the soil arrests all nitrogen fixation for a considerable length of time, namely until the raw materials are completely digested. Such composts have a depressing effect upon the nitrogen level in soils similar to the effect of green manuring. Their value begins to show up only after many months, often only during the second year, as is the case when one uses fresh manure. Applications of these materials should be made in fall, never in spring just before seeding or planting. The same holds true for sheet composting of organic wastes and trash, and for partly rotted straw and sawdust compost. Well-rotted composts can be applied at any time although a moist soil is a better recipient than a dry one. Desert remains desert as long as there is no moisture.
Completely mineralized composts are the other extreme and cannot be called ‘organic fertilizers’. In completely mineralized composts, however, the minerals are usually in a high available form while in partly-rotted, raw composts they are usually unavailable.
From all these facts it appears that the proper degree of breakdown in a compost is important in determining the effective amounts to apply. If a compost with 2 to 3% nitrogen, but not well decomposed (i.e. more or less raw) is applied most of the nitrogen is lost in the soil. One would fare much better with the 1% nitrogen of a well-rotted compost. In the latter, the nitrogen will be slowly and steadily made available to plant roots and the microlife in soil. It should always be remembered that we feed the microlife first and only secondarily the plant root. It is the microlife in soil which turns the minerals into forms which the plant roots can take up, or not, as the case may be. (2) (3).
An ammonia odor in compost or manure indicates the presence of free ammonia. Since ammonia has a strong smell its presence is not always an indication of a high ammonia, i.e. nitrogen content. Fresh manure, after a few days, can really stink of ammonia and yet contain only 0.5% nitrogen. The ammonia odor, however, indicates that the ammonia is free, that it can easily be lost or washed out and only that much is used as the plant roots can absorb at once. The balance is readily lost unless there is nitrification or other fixation process going on in the soil. When the manufacture of compost from garbage was started and the product sold to farmers, we were especially proud of the fact that the compost had an earthy, agreeable smell. Some of the salesmen came back and reported that we should give it a stronger odor because the farmers said that it didn’t have ‘that strong fragrance of manure’. We thought that in the interest of nitrogen preservation and effectiveness it would be better to perfume the compost than to permit a decomposition with a strong ammonia smell. Without nitrogen fixation ammoniac composts have an immediate, but not lasting effect on plant growth. The plants may grow too lush and go to seed too early (under drought conditions), or set weak seed (under wet conditions), for most of the ammonia nitrogen is lost long before the plants mature. Too much of an easily available form of nitrogen may even lead to burns on plants as can be seen when fresh poultry manure or too much concentrated urea, is applied to lawns.
Phosphates and potash are present in composts in both organic and inorganic forms, and are made available by soil bacteria and their enzymes no matter whether the chemical laboratory tests show them to be present in available or water soluble form or not. It is, therefore, in the interest of the compost producer as well as the compost user to create such conditions that an active microlife can be utilized. This is only possible in the presence of moisture, organic matter (the more the better), and aeration, that is, proper cultivation. The release of these elements in the soil is also slow and steady. In addition, in soils which have been treated intensively with fertilizers in the past, the increased microlife will bring tied down phosphates and potash back into circulation. The low P and K in composts (which usually range from 0.5 to 1.5%, in the best products up to 2%) are no obstacle since maximum effectiveness and availability are obtained. Again it should be remembered that the application rate for composts is quite different from that recommended for concentrated inorganic fertilizers. If 1 ton of compost at 1% P or K is applied to an acre this means that 20 lbs. of the corresponding element are given. Ten tons of the same compost contain 200 lbs., which in most cases is sufficient. Dry B.D. Compost Starter treated composts are actually concentrates, and the 2% phosphates or potash in them provide 40 lbs. of P or K per acre at the rate of 1 ton/acre – all available in due time, and no losses or tieing down are encountered.
It is our firm opinion, based on laboratory and field tests as well as on many years of practical application (4), that composting can be done very well along scientific, analytical lines. The first step is to know your compost and what it contains. You ought also to know your soils and the crop demands. Finally you ought to know the crop rotation and take into account the beneficial influences of legumes and green manuring.
A soil analysis will tell which elements are abundant and which are lacking or deficient. But chemical information about your soil is not the only important thing. It is the physical structure of the soil and the climate or weather which play a decisive role in the efficient and effective use of composts. Light, sandy soils can absorb large amounts of compost until they build up humus – organic reserves. Once a 2% organic matter level is reached they will make full use of compost as fertilizer. Below a 1.5% organic matter level part or all of the compost applied will be used either for soil building (when the compounds in the compost become available slowly), or as fertilizer (when the compounds are readily and quickly available ones). Soils below the 1.5% organic matter level have no reserves and live from day to day.
Medium-heavy clay soils with better than 2% organic matter are the ideal soils. Very heavy clay or loamy soils react sluggishly and large amounts of manure or compost have to be applied before you can see any results at all. However, these amounts are not lost, they are stored. It is more important to aerate and drain heavy soils according to the best knowledge of agriculture to make the best and most effective use of composts. These in turn aid in loosening up heavy soils and improve the structure. Once the structure is improved the full effectiveness of composts as fertilizers will be apparent. In a garden raised beds, for truck and field crops furrow cultivation, will greatly improve such heavy soils.
The higher the organic matter level, the better the soil structure, the smaller the amount of compost which is needed. Good drainage is a must for efficient compost use. In wet, heavy soils no compost should be buried deep. The climatic conditions and weather also have a bearing on the rate of compost application. The general rule is that heat and drought destroy organic matter faster than cool and rainy climates. In hot, dry climates, as well as on light and heavy soils, it is more efficient to use smaller amounts and apply them more often. This practice is recommended, by the way, for all difficult soils. In such cases, yearly applications are preferable, while on well-maintained soils one heavier application once in a while (once in a four-year crop rotation, for instance) is usually more practicable.
Last but not least, the crop demands and crop rotation need to be considered. A general differentiation should be made between plants with high, moderate or low nutritional requirements. In using compost where we have a natural balance in the soil one can take a chance and think in terms of tons per acre, even though the NPK details are not know. However, to this writer, the full application of all possible information is always preferable. A commercial farmer should use all the information; the home gardener can risk getting away with a general application.
Heavy nutritional demands on soil are made by the following: Corn, tomatoes, potatoes, beets, peppers, cucumbers, spinach, chard, cabbage, cauliflower, endive, escarole, chicory, celery, squash, pumpkins, okra, rhubarb, sunflowers, roses, peonies and some other perennial flowers, berries, heavy-bearing fruit trees, nut trees, avocados, bulbs, lawns.
Plants which make moderate nutritional demands are: Lettuce, carrots, turnips, early garden beets, Brussels sprouts, kohlrabi, leeks, onions, most spring flowers, pasture and hay grasses, small grains, in general, shallow-rooting plants.
Light demands are made, especially in fertile soils with a high organic matter content, by radishes, most herbs (medicinal herbs in particular), legumes (peas, beans, clovers, alfalfa and soybeans if allowed to mature), most summer and fall flowers, buckwheat, tall pasture and hay grasses with deep growing root systems. To be on the safe side it may be stated (with a grain of salt) that light feeders turn into heavy feeders on light, sandy soils and any soil with a low organic matter content. Such soils should be given large quantities of compost and frequently.
RULE II: Adjust your compost to the soil and climate.
RULE III: Adjust your compost to the kind of crop you want to grow.
For poor conditions and heavy feeders use the maximum amount of 10 tons/arce of a good grade, wet compost (0.5-0.7-0.5 formula), or less, according to formula, of a dry, manufactured compost. The compost manufacturer should be in a position to advise the customer as to the proper use of his product. Unfortunately, this is not yet true in the case of most of the products advertised nowadays. These products may have a 2-2-2 formula at less than 20% moisture, then 2 tons/acre will be enough. Use 2 to 3 tons of 1-1-1 formula at less than 20% moisture. These rates apply only to products which have an organic matter content better than 20%. Manufactured composts with 12-16% organic matter, as well as manures, have to be used at the maximum rate.
For good conditions and moderate feeders use half the rate of maximum demand, for instance, 5 to 7 tons of a moist, homemade compost or 1 to 2 tons in the field. On good soils after a heavy or moderate feeder, compost can be omitted. On poor soils a light dressing is advisable.
For light feeders, minimum amounts can be used; a light sprinkling in a garden, 1 to 2 tons in the field. On good soils after a heavy or moderate feeder, compost can be omitted. On poor soils a light dressing is advisable.
The crop rotation in garden and field with compost management, on good soils, is best arranged in such a way that heavy feeders are provided with maximum amounts to be followed by moderate feeders with half amounts on good soils, to be followed with light feeders and no compost. On poor and difficult soils the old power of maximum amounts will probably last into the second year. Therefore, in such cases, it is better to use light feeders in the second year of the rotation with a light application of compost. This is followed in the third year by moderate feeders with a half rate application. In the fourth place in the rotation one can then again place the heavy feeders and apply the full amount once more. On good soils, in good condition, either a light or moderate feeder can be grown in the fourth year, before one returns to the heavy feeders with a full application. These rules should not be adhered to stubbornly, but used with common sense. Modifications are always possible. And we have not included in these possible rotations cover and rest crops, or rest periods.
Mulching can be done with a poorer grade of compost which has not become earthy, that is an incompletely rotted compost. On dry and light soils and in hot dry climates, it is a must.
Green manuring crops, mown and top-dressed with a light application of compost, even of a poor compost, will be twice as effective as those not so treated. The plowed or disked in material will decompose faster and release its nutrients quicker without tying down nitrogen.
Thus far we have been dealing with the basic principles of composting and their ordinary practical application. A further development of these scientific principles in even more detail will form the basis of future articles. But on the basis of studies already made, it is evident that composts can be applied scientifically just like any other fertilizer, and application rates and the types of compost to be used figured out for any purpose and requirement.
The two tables printed on the folded insert will help you in working toward this scientific, economic, and effective use of your composts. The first table brings together all the information from the text of this article which will help you in determining how much of the various types of compost to use in general practice. With the help of Table II you can adjust your applications to the individual needs of any crop.
Our aim is to take the guesswork and happenstance out of composting and to make it as exact an agricultural science as possible. Farmers and gardeners who have done this have fared well over long periods of years. One observation, frequently made and confirmed, is quite important in this connection: the usual application rates especially of low grade composts have more than provided all the requirements of crops without causing any trouble in regard to soil structure.
(1) HOW MOIST IS MOIST? By E. E. Pfeiffer,
BIO-DYNAMICS, Fall & Winter 1953, Vol. XI< Nos. 4 & 5
(2) FEED THE SOIL by F. L. Wynd,
THE SCIENCE MONTHLY, Vol. LXXIV
(3) SEVEN YEARS OF SOIL SURVEY ON A BIODYNAMIC
FARM by E. E. Pfeiffer, BIODYNAMICS, Summer, 1954,
Vol. XII, No. 3
Reprinted from BIO-DYNAMICS Fall Issue, 1954
Copyright 1954 by the
Bio-Dynamic Farming and Gardening Association, Inc.
Table I - Average Compost Application As A Guide to Practice
Table II - Formula and Application Relationship: For Orientation, and for Individual Application of Composts
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