Chapter 5. Soil Fertilizers Uses and Sources

A common illusion about fertilizers is that they are direct nutri­ents which must be promptly absorbed or they will be lost (either leached out of the soil by rain, or locked up in some insoluble, un­usable form). The fact is that instead of being used directly to any extent, most of the nutrients in fertilizer compounds are quickly blotted up by soil organisms such as bacteria, fungi, protozoa and actinomyces. Gardener's Loam contains billions upon billions of such organisms which occupy the soil mass so completely that very little in the way of soluble plant food can escape them. They act as reser­voirs of fertility, releasing it when they die. Since their life span is short, such food is not tied up for long periods, but merely saved for later use by plants. The role of microorganisms in conserving fertil­ity cannot be overestimated.

The ideal fertilizer would be one that supplied every needed el­ement of nutrition for the crop being grown, at a rate that would take advantage of all available light, heat, moisture and oxygen needed by plants. This ideal material would supply some nutrients in quickly-available form for immediate growth, yet would contain other nutrient fractions that would be released so slowly that a single application in spring would continue to feed until cold stopped plant growth in fall. All through this cycle of combined slow and fast re­lease, enough surplus nutrients would be given off to provide food needed by soil organisms to carry on their functions.

Claims for various materials are endless and often self-contradic­tory. In general it may be said that chemical and mineral fertilizers are more readily available for immediate use by plants, while organic materials (which must undergo more involved decomposition before they can be absorbed) are slower in action and last longer in the soil. These distinctions are being gradually eliminated by the develop­ment of long-lasting chemicals that release fertility even more slowly than some organic manures.

Fertilizer Burn

The claim most often made for organic fertilizers is that they "do not burn." By burning is meant injury (dehydration) of the roots or crown of the plant, and a browning of part or all of the foliage, some­times resulting in the death of the entire plant. The tissues are not "burned" as in a bonfire; the burn is a drying-out caused by the withdrawal of water from tissues by the hygroscopic action of "thirsty" chemical materials. Since some manufacturers make a big issue of this "non-burning" quality, many home gardeners have come to regard it as an important factor in selecting a fertilizer product for use.

When anyone makes the claim, "This fertilizer won't burn," my immediate reply is, "Then it won't feed, either." The identical prop­erty which makes food elements available as plant nutrients—high solubility—will also make them likely to "burn" or injure plant tissues. The burn may be immediate or it may be delayed, but if the soluble ingredient which causes this condition is present in excess amounts, burn is almost inevitable.

A Sod Story

On lawns, for example, a burn from an overdose of a chemical fertilizer such as ammonium sulfate becomes visible almost at once, while a too-heavy application of sewerage sludge may not show any ill effects for a month or two after application to the turf. Unfortu­nately, the burn from organic sources comes so much later after application that it is seldom traced to its cause. Nevertheless, be­cause it happens in summer—a time when grass is peculiarly suscep­tible to severe injury—a delayed organic nitrogen burn of this kind usually causes more permanent injury than does a burn inflicted early in the season by a chemical fertilizer.

To understand how all this happens let us look at two lawn areas, one fertilized heavily with a highly soluble chemical like ammonium nitrate and the other with sewerage sludge (a typical organic fer­tilizer). In the first instance, the owner applies 10 pounds of ammo­nium nitrate in spring soon after grass begins to grow. He fails to water it in, and so the chemical fertilizer begins to suck moisture out of the grass plants to satisfy its "thirst." In a matter of a few days, the entire lawn is dappled brown and green—severe nitrogen burn. After the owner realizes his mistake, he waters heavily to wash out the excess salts; the grass gradually recovers and turns dark green. By June it is healthy and ready for another feeding. This time the owner waters it in and no burn results.

The same week in early spring the second lawn, perhaps next door to the first and on the same type of soil, is fed with sewerage sludge because the owner is convinced this organic fertilizer will not burn. He applies 50 pounds per 1,000 square feet of lawn. April and May remain cool, however, and because soil bacteria are inactive the or­ganic material is not broken down and this first application produces no results. The owner, certain he needs more fertilizer, dumps on another 25 pounds of sludge per 1,000 square feet. The two applica­tions contribute less than four pounds of actual nitrogen per 1,000 square feet, not an excessive amount by any means. If organic nitro­gen were really non-burning, he would have no trouble.

Unfortunately for him, the theory doesn't hold water. About the first week in June, hot weather comes on suddenly. Soil bacteria that have been all but dormant begin to multiply at an unbelievable rate. In a matter of two weeks, nitrogen is being released from the sludge-rich soil to the plants so rapidly that grass blades wilt and collapse. If a nitrogen test chemical is applied to the grass at this time, tiny beads of red will show up on every blade. This indicates that free nitrogen is being released—actually exuded from the blades—by a process known as guttation. This lawn is suffering from a far more serious nitrogen burn than lawn number one received in spring. About as close as the second lawn owner will come to knowing the real cause, however, will be his comment, "I guess my grass just couldn't stand summer heat."

Golf course greenskeepers are familiar with this problem. Because they must always maintain grass in top condition, they often have to push turf feeding close to capacity and risk bringing on guttation. The moment they see traces of grass blade wilting or "flagging" they will apply test chemicals to see if nitrogen is coming out of the foliage. If it is, a crew of men is quickly assigned to wash (leach) the excess plant food out of the soil with liberal applications of water.

Avoid Excess

A paramount fact every gardener should fix in mind is that any excess of nitrogen beyond the needs of the plant will cause a burn. This burn may become visible very soon after the fertilizer is applied or, in the case of manures, the effect may be delayed many weeks.

Even certain organic products can cause rapid burn if used in excess. Dried blood, one of the most valuable sources of nutrients, is so readily soluble that it may work like a chemical fertilizer salt. The same is true, to a somewhat lesser degree, of fish emulsions and urine.

Buy by Content

In the following source lists of fertilizers (and throughout the book) you will note that I almost invariably give technical names and discuss basic fertilizer materials rather than trade-named prod­ucts. Every ingredient can be found in a variety of trade-named "plant foods." Fancy names and pretty pictures on the fertilizer bag won't feed your plants. Read the legally required label of contents on the bag or box of each brand of fertilizer in the store. Buying accord­ing to actual content of ingredients is the only sure way of getting what you want, and need—and pay for.

CHEMICAL SOURCES OF NITROGEN

Ammonia (liquid ammonia): This is perhaps the most widely used of all nitrogen fertilizers today, yet is of no practical value to the home gardener because special apparatus is needed to apply it. It is mentioned here only because many gardeners ask about it after read­ing accounts of its use in agriculture. These liquids run about 30 per cent ammonia, of which about 85 per cent is nitrogen.

Ammo-Phos (ammonium phosphate): There are two commercial grades of this material. Grade A contains 11 per cent nitrogen and 48 per cent available phosphoric acid. Grade B contains 16 per cent nitrogen and 20 per cent phosphoric acid. Both are excellent sources of completely soluble nitrogen and phosphorus.

Ammonium Phosphate: There are two grades: monoammonium phosphate contains 11 per cent nitrogen and 60 per cent phosphoric acid while diammonium phosphate analyzes at 23 per cent nitrogen and 53 per cent phosphoric acid. Both are completely soluble. Be­ware of using them on rhododendrons and other acid-loving plants, however, as they are quite alkaline in reaction.

Ammonium Sulfate (sulfate of ammonia): Once the leading source of nitrogen in chemical fertilizers, it is still #1 on the home garden­er's list. In the agricultural field its place is being taken over by liquid ammonia. Sulfate of ammonia contains about 20 per cent nitrogen. It can be applied dry but must be watered in immediately to avoid burning. It is much safer if first dissolved in water.

Sodium Nitrate (nitrate of soda): This was one of the first chem­icals used as a fertilizer. Vast deposits of sodium, combined with oxygen and nitrogen, were found in Chile, and were worked for fer­tilizer purposes during the nineteenth century. Because it was for many years the leading source of chemical nitrogen, it is firmly entrenched in the literature of gardening. It is often recommended out of habit when other materials would be safer and better. Sodium nitrate may have some use in strongly acid soils but will deflocculate clays and make them greasy if used too often. It is not an ideal source of nitrogen and other materials should be substituted if possible.

Urea: Discovered originally in urine, this is now produced syn­thetically in large quantities. Urea is not a protein but because it contains a carbon particle, it is classed as an organic compound, to the consternation of organocultists. While not instantly available, urea goes through fewer stages to break down into nitrate form, hence starts feeding a little more rapidly than do organic products.

Ureaform: This is a most unusual fertilizer material—it is best described as a nitrogen-bearing soft plastic material which breaks down slowly but uniformly when in contact with soil organisms and moisture. It is made by reacting urea and formaldehyde. It contains 38 per cent nitrogen, yet can be applied to grass without fear of burning. This is because ureaform gives off nitrogen so slowly that grass can absorb it about as fast as it is released. Enormous quanti­ties have to be applied before a burn can be produced. Ureaforms combine the best features of chemical and organic plant foods. Their one drawback is the slowness with which they begin to feed. (See rec­ommendations under mixed fertilizers for overcoming this weakness.)

ORGANIC SOURCES OF NITROGEN

Any organic material which contains protein can be considered a source of nitrogen, but whether it will be economical to use is another question. A friend of mine once wanted to set up a factory to process garbage for use as fertilizer. In a day's time, I located thirty other waste products for him in his city which would give him a higher return for his efforts than garbage. Many of these products could be had free and all were more pleasant to handle.

Organic products which are high enough in nitrogen to be worth commercial development are not too easy to find. Here is a list of some which are generally available:

Castor Putnace: This is the refuse left after castor beans are proc­essed for oil. It cannot be used for cattle feed because it is poisonous to animals (but not to plants). It contains about 5.5 per cent organic nitrogen. Traces of both phosphorus and potash make it a fair fer­tilizer, particularly on acid-loving plants.

Cottonseed Meal: Also used as a fertilizer for acid-soil plants, it contains about 6 to 7 per cent nitrogen, 2 per cent phosphorus and 2 per cent potash. Since it can be used for cattle feed, the price is usually too high for general garden fertilizer use.

Dried Blood: Perhaps the most valuable single fertilizer available —organic or inorganic—because it contains in quickly soluble form every element needed by plants for growth. Only the cheaper grades of dried blood (which contain about 9 per cent nitrogen) are used as fertilizers, however, since the better grades are used for industrial purposes and cattle feed and thus command high prices. Fresh blood, sometimes available from local slaughterhouses or from poultry processing plants, can be adsorbed on peat moss, vermiculite or similar materials and used in that state. Nothing gives foliage plants as fine a dark green color as does dried blood.

Fish Emulsions: These fertilizers are produced by soaking trash fish, offal and scraps in water to extract all the solubles. This extract is then condensed until it contains less than 50 per cent water. Sur­prisingly, the condensed product does not have an offensively fishy odor. The method of extracting insures that all elements are present in soluble form and are readily available to plants. Like dried blood, fish emulsions (they contain considerable blood) provide every el­ement needed for growth. In my experience they are ideal for shade-loving plants like tuberous begonias, gloxinias, African violets, and so on. Fish emulsions have a nitrogen content of about 5 per cent, but they should not be judged solely on nitrogen.

Sewerage Sludge: Perhaps this is the most widely used of all or­ganic fertilizers for lawns. Activated sludge is a black, flocculated organic material produced by treating solids in sewerage and allow­ing them to settle out in special beds. If the nitrogen content is more than 5 per cent and the analysis shows any amount of potash, the chances are that the sludge has been doctored with additional chem­ical nitrogen and potash. Activated sludge is a good conditioner for other fertilizers that tend to cake in the bag, hence it is used to a far greater extent than most gardeners realize.

Tankage: This is made up of packing house wastes steamed to extract the animal fats. The remaining tankage contains between 6 and 10 per cent nitrogen.

PHOSPHORUS SOURCES

Bone Meal: This is possibly the most overrated of all fertilizer materials. Beloved by tradition-bound British gardeners, it has been the universal remedy recommended whenever the "authority" was stumped and had to say something. I suspect the reason bone meal is so frequently recommended is that since it does nothing to the plant of any importance, it does no harm. In fairly acid soils, for example those with pH readings of from 5.8 to 6.2, phosphorus becomes avail­able if bone meal is used liberally. In soils of higher or lower pH readings, phosphorus locks up in insoluble forms that cannot be used by plants. Where phosphorus is needed, superphosphate will supply it at a fraction of the cost of bone meal, and in much more available form. If bone meal can be had practically for nothing it has some slight value, largely for its small content of nitrogen.

Rock Phosphate: In raw form, just as it is dug from the ground and pulverized, rock phosphate is a fairly good source of slowly available phosphorus. The studious gardener who consults foreign texts should not be deceived, however, by results reported in Europe. Phosphate rock used abroad comes from Africa and is of a different type than American rock. The African product provides much more available phosphorus. Finely ground American phosphate rock, in acid soils, becomes slowly available after the second year. In alkaline soils, it is practically worthless.

Superphosphate: This is the basic phosphorus fertilizer. It is a mixture of monocalcium phosphate and calcium sul ate, produced by treating the raw rock with sulfuric acid. The regular grade contains about 20 per cent phosphoric acid, while triple super phosphate may go as high as 48 per cent. Someone has said that without this source of phosphorus, American agriculture would grind to a halt. While this is a bit extravagant, the statement does point up the vital role played by this one material.

POTASSIUM SOURCES

Muriate of Potash: It's odd how this old-fashioned name remains in use! Muriate comes from Muria, the Latin for brine. Muriate of potash is potassium chloride containing between 50 and 60 per cent potash. It was deposited eons ago by ancient seas and should be considered a natural product, blessed by organocultists, but it is not. Its chlorine content passes off rapidly when applied to soil. As ex­plained under soil organisms, however, muriate of potash is harmful to certain beneficial bacteria. Some authorities think sulfate of potash is better.

Sulfate of Potash: This contains 48 per cent potash. It is more expensive than muriate of potash but is considered less harmful to bacteria and plant roots.

Wood Ashes: About the only generally-available organic source of potash, this material is treasured by organic gardeners. Wood ashes contain about 6 per cent potash, plus considerable lime. Before corn cobs were used industrially, the cobs were burned in huge piles. The resultant ashes were peculiarly rich in potash—up to 35 per cent. Almost any ash resulting from burning organic materials that contain some fiber should be a fair source of potash. Wood ashes are partic­ularly good to use for adding potash to a compost heap.

MIXED FERTILIZERS

Most home gardeners prefer to buy their plant nutrients as pre-mixed products—the so-called complete fertilizers (often errone­ously called balanced fertilizers). Many years ago, before we knew as much as we do today about plant nutrition, three elements were said to be essential. Although we still know too little about nutrition, we do at least realize that these "Big Three"—nitrogen, phosphorus and potash—by no means supply all elements vital to growth. Never­theless, any mixed fertilizer containing these three elements may legally be labeled "complete." According to law, at least in most states, such complete fertilizers must contain at least twenty units of plant food. Figures that state how many units are contained in a product must appear on the bag or package, with nitrogen, phos­phorus and potash appearing in that order. In one or two southern states, however, the figures for the last two elements are sometimes listed in reverse order.

How to Read the Bag

So far we have considered the nutrient content of fertilizer mate­rials in percentages. If you want to apply 4 pounds of actual nitrogen to 1,000 square feet of lawn, it is easy to figure that this can be supplied by 100 pounds of a 4 per cent material or 25 pounds of a 16 per cent material.

The problem seems more confusing when mixed fertilizers are used because the bags carry three figures instead of one. However, remember that these are still percentages. Thus a 10-8-6 fertilizer contains 10 pounds of nitrogen (N), eight of phosphorus (P) and six of potash (K) in every 100 pounds of fertilizer.

When I first began using commercial fertilizers, figuring percent­ages was easy: everything was packed in 100 pound bags. This was hard on the back, of course. We had no handy small cartons, no 50- or 35-pound bags, but at least we knew, without figuring, that a 100-pound bag of 5-10-5 would supply 5 pounds of nitrogen, 10 of phosphorus and 5 of potash.

Today it takes a wizard to figure out actual weights and percent­ages. Despite the need for doing a little paper work, the gardener who wants to be careful with his pennies should take time to work out costs.

Cost per Pound

The most important figure is the cost per pound of nitrogen. A product which contains 40 per cent or 40 units of nitrogen and costs $20 per 100 pounds sounds expensive by the pound (50 cents per unit). It would, however, be cheaper than sheep manure which sells for $2.10 for a 50-pound bag but contains only between 1 per cent and 2 per cent nitrogen. Compare the 50 cents per unit cost of nitro­gen in the first product with the cost—between $2 and $4—of each nitrogen unit in the second product, sheep manure.

Another way to figure costs is by the total plant food in a product. A dried sheep manure, for example, that contains 2-1-1 units of the "big three" nutrients, usually sells for $2.10 for a 50-pound bag; this sounds cheap, yet the cost per nutrient unit is over $1. A mixed urea-form fertilizer analyzing at 20-5-5 and selling for $9.95 for a 50-pound bag would at first glance seem many times costlier, yet the cost for all the units is less than 67 cents per unit.

In figuring fertilizer costs you should give some thought to the form in which the plant food occurs. While urea would be cheaper on a cost-per-unit basis, it is inferior to ureaform as a long-lasting turf fertilizer that provides an eight-month feeding period and greater safety in application.

Rates of Feeding

A great deal of fuss is made about the feeding value of various fertilizer formulae. I have seen amateur rose growers, for example, all but come to blows in arguing whether a 6-1Q-7 was a better rose fertilizer than a 5-10-5 or a 4-12-4. Advocates of all three were vehement in their protests that only their ratios and rates of feeding would produce perfect roses.

I wish I could be that dogmatic with any degree of confidence in my recommendations. Where I can be dogmatic is in saying that there is no perfect general formula. Within reason, any complete fertilizer will produce good roses and other plants if the grower uses enough, without going overboard. Soil is an amazing buffering agent and will accept many times the amount of fertilizer usually recom­mended—without injury to the plants. I recall a vegetable garden I made early in World War II. As a member and officer of the Illinois State Victory Garden committee, I was on the go night after night and had little time to do my own work. Yet I felt that I should set a good example and make a home vegetable garden. The only answer was to have the soil preparation and fertilizing done for me. The handy man I hired was not too good at figures (particularly dec­imals) and skipped a place in his calculations, which resulted in the application of ten times as much 10-6-4 fertilizer as I had intended.

Fortunately, this was a complete fertilizer, not only in the big three of N, P, and K, but in the minor elements as well. To show how selective and discriminating plants are in their food uptake, I saw not one instance of over-feeding of any one element. True, the weed growth was phenomenal (this was the first time in my life I ever had to cut ragweed with an axe) but everything else grew on the same phenomenal scale. So long as food elements are in balance and are supplied in amounts sufficient for good growth, with no one ingre­dient lacking, plants can be depended upon to take what they need and leave the surplus unused.

This does not mean that the point cannot be reached where the soil will be saturated with excess soluble salts which will damage or kill the plants. It does mean that the soil, particularly when it is liberally supplied with organic matter, is capable of buffering tre­mendous overdoses, so that, within reason, an accidental overdose need not be a calamity.

Take Your Choice

You may think I am inconsistent in recommending that formulae be checked carefully to see that plant food units are being purchased at the lowest cost, then stating that not too much attention need be paid to the value of one formulae over another, providing they are reasonably similar in analysis and seem suited to the use to be made of them. My reasoning is, however, sound.

In order to determine exactly which of two or more fertilizer formulae would better meet the needs of the flowers, vegetables, shrubs or fruits growing in the garden, you would have to: (1) make expensive soil analyses at intervals during the growing season, to see what is left from the nutrients you applied, and (2) run experiments to see whether variations in the fertilizer formulae would produce superior results at lower costs. In the end, all this bother might save you a dollar or two a year, but this saving would be offset several times over by the cost of the soil analysis. I feel that soil tests for gardening is like swallowing a camel but straining at a gnat.

Of course, the technically minded gardener who enjoys dotting his Is and crossing his Ts can't fully enjoy himself without soil tests. I just don't feel they are necessary or desirable for the "average" home gardener.

Incidentally, a generally overlooked point concerning organic fertilizers (other than urea or urea-form products) is their con­tent of insoluble nitrogen. Unless all or almost all of the nitrogen in any organic product is insoluble in water, the product will not give the "slow-release" or slowly available nitrogen effect for which you purchased and applied it. Here, again, is a dandy reason for checking package labels—including the tiny print—before you buy.

Chapter Digest

The ideal fertilizer—one that is all things to all plants—probably never will be produced, either by nature or by science. But there are many chemical and organic fertilizer source materials which supply nutrients that will do a good job if properly used, especially in mixtures. The discussion of nitrogen fertilizer "burn" should clarify—and eliminate—the problem for all gardeners. Similarly helpful are the explanations of specific fertilizer elements, how plants utilize them, the meaning of the three numbers on a fertilizer bag, and ways to figure fertilizer costs.