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Fertile Soil: Understanding The Soil Fertility Puzzle

28 de februar de 2021

In this article, we are going to look at soil fertility.

Mention “fertilizer” to an engineer or a chemist and they begin immediately to think in terms of inorganic minerals which have been assembled into a bag which their mind labels “commercial fertilizer.”

plant poking out of a puzzle growing in fertile soilTalk of fertilizer to the organic gardening zealot and they see visions of:

  • Compost heaps
  • Bone meal
  • Manure
  • Dried blood

… and other natural sources of nutrients.

The poor old home gardener is trapped between a barrage of, get this, OPINION, are from these two schools whose logic is about as valid as two plus two equals five.

Soil Fertility Through A Soil Organism Lense

Let’s look at soil fertility from a soil organism’s point of view.

First of all, recall the rock particle structure that is the skeleton of all soils:

  • Rock
  • Gravel
  • Sand
  • Silt
  • Clay

… all mixed together and all composed of broken up rock.

Between these different sized bits of crumbled rock there is:

  • Air
  • Water with dissolved mineral and organic nutrients
  • Humus which is dead organic material
  • Living organic material such as worms, grubs, protozoans, algae, fungi, plant roots and so on.

Now let us examine where nutrient materials come from in all this mixture.

We have a living root that will take in nutrition for the plant.

Where does that nutrient come from?

Since all food material must be dissolved before it can enter the plant, the only available food is that which is found in the soil solution.

How did it get there?

Several possibilities come to mind.

  • Someone may have fertilized the garden and the minerals have dissolved in the rain water and washed down around the roots. This is one possible source.
  • Another possibility is that leaves have fallen, rotted down, being gradually digested by the living organisms of the soil until they reach the soluble state.

This is the usual story in the undisturbed forests and prairies. There is a cycle of growth, death, decay, solubilization and regrowth.

  • The third possibility is that animal fertilizer has been scattered on the garden and the nutrient ingredients have leached down through the soil.

Why does it make a difference whether a gardener uses commercial fertilizer or manure?

The Humus Factor

The answer lies in the humus fraction of the soil.

Any gardener knows that a purely mineral soil composed only of clay, silt, sand and gravel is fairly sterile, unretentive of water and a poor soil to try to garden on.

They would improve the soil by adding humus – then the soil would have a better texture, hold moisture, cultivate more readily and certainly produce better plants.

The secret, then, lies in the humus content.

How Can Humus Affect The Available Nutrients In The Soil?

Humus is the fluffy, brownish residue from decaying plant and animal bodies. As it breaks down, nutrient materials are released into the soil.

This breakdown is caused by soil microorganisms, both plant and animal.

Any very healthy soil has a fantastic population of microbes living in it.

These tiny creatures are responsible for the steady breakdown of humus to its soluble components which then become available for plant nutrition.

The whole key to a healthy soil is its microbial population and this is affected chiefly by two factors, aeration and humus content.

cross section of fertile soil with earthwormsAt this point we begin to see the organic gardener’s viewpoint.

Humus, derived from compost, animal manure and mulching is an essential ingredient in good soil.

We can see, too, the fallacy in the practice of using nothing but chemical fertilizers in the soil, since they stimulate both plants and microbe growth, actually hastening the depletion of humus from the soil and thus ruining it.

But there is another side to the picture, too.

Our soils are losing certain essential minerals through leaching (washing out with water from rainfall, flooding and so on), through removal cropping and through poor cultivation practices.

Unbelievable amounts of manure would be required to sustain the crops we wish to produce, whether it is a field of corn or a bed of annuals.

In some European countries it was the custom to top-dress the fields with as much as six to eight inches of manure a year, water the crops with the seepage fluids from the barnyard and manure pile, and still the soils become worse.

Commercial Fertilizer vs Organic Gardening Feud

The answer to the commercial fertilizer versus organic gardening feud is a compromise. Both are necessary to a program of soil improvement.

Now we see a systematic complex with a steadily available flow of soluble plant nutrients coming from a number of sources.

They may be released from clay particles, they may come from digested humus or they may seep down from a top-dressing of commercial fertilizer, but, and this is extremely important.

If we are to keep the soil healthy and in good shape, the nutrient level remains constant, the humus level remains constant and the soil texture remains friable.

It is a good gardener who can manipulate their fertilizing and mulching practices to achieve this sort of thing.

Building The Soil

There are some worth-while rules of thumb to help a gardener build up their soil.

Do deep spading in fall and early winter. At other times, if air is stirred deeply through a warm soil the immediate burst of bacterial growth badly depletes the humus.

Try to have some humus, manure or even fresh plant debris to throw into the bottom of the trench. You can deepen your topsoil this way.

Plan to use ample amounts of chemical fertilizer in the very early spring.

Just as salt can kill your shrubs and grass, so soluble chemical fertilizer can damage your plants when they are actively growing.

On the other hand, by applying soluble fertilizers in the early spring when the plants are dormant and fairly burn-proof, the chemicals can leach downward, be picked up by the clay particles and held for later use.

Of course this will not work well on an overly sandy soil.

Use a system of shallow cultivation and generous mulching through the summer months.

But remember, mulch is food for microorganisms and they will reproduce rapidly when it is used. In making new protoplasm they use lots of nitrogen.

This is why gardeners sometimes see a recently mulched planting turn yellow. The lesson is simple. Dust a small amount of chemical fertilizer over the mulch and keep everyone happy.

And. finally, take a page from the old English gardener’s notebook. Try mixing a small bucket of balanced chemical fertilizer with a wheelbarrow full of compost and top-dress your plants with this mixture.

The results are beautiful to see and you have the satisfaction of knowing that your soil will be better than ever.

What is Meant By Soil Fertility

handful of soil for the gardenSoil fertility refers to the available nutrient materials found in soils.

Soils may be heavy with clay or sandy and porous.

They may be soggy or well-drained, mineral or fluffy with humus, but the ultimate criterion of their ability to support plant growth must be judged on the basis of fertility level.

It is true that plants, through photosynthesis, manufacture carbohydrates that are further metabolized into various structural materials, proteins, fats, oils, and so on.

The Major Mineral Elements

But every plant requires substantial amounts of mineral nutrition, and these minerals are derived from the soil.

A fertile soil is one that has an ample supply of available minerals plants need in order to reach their optimum growth.

Some minerals are used by plants in relatively large amounts.

It has become customary to refer to these minerals as the major mineral elements.

The most important major elements are nitrogen, phosphorus and potassium.

Any bag of fertilizer sold in the United States is required by law to have on it three numbers, always in a certain sequence.

You may find 5-10-5 or 10-6-4, two very common formulations.

  • The first figure is the number of pounds of nitrogen in a 100 pound bag
  • The second is the number of pounds of phosphorus in 100 pounds
  • The third figure is the number of pounds of potassium in 100 pounds.

More on –> NPK fertilizer ratio

Of course, the nitrogen may be in several different forms, as may the other nutrient elements.

The question always is asked:

“If the total nitrogen, phosphorus, potassium poundage adds up to, say, 20 pounds, what is the other 80 pounds in the bag?”

That is where it becomes necessary to study the chemistry of soil nutrients.

Let’s say the measured nutrient elements are necessarily combined with other minerals, many of which are useful to the plants, too, and these complete the fertilizer poundage.

By way of example, nitrogen may be present as a mixture of:

  • Ammonium nitrate
  • Ammonium sulfate
  • Ammonium phosphate
  • Calcium nitrate

… and possibly in organic combinations of blood meal, tankage and cottonseed meal.

What The Best Fertilizers Include

In fact, the best fertilizers often include the same mineral element in several different forms to give a slow, steady release of fertility in the soil and thus maintain better plant growth.

Each mineral element is used in a special way by the plant that takes it up.


Nitrogen is used in huge quantities because it is a chief constituent of all protein, and therefore, a basic requirement for the protoplasm of all living tissue.

Nitrogen appears also in other important organic compounds in plants.

When nitrogen is lacking, plants are likely to be stunted and crippled. They will be yellowish and unhealthy looking.

Most plants take in their nitrogen as nitrate but some may be able to utilize ammonia as a source of nitrogen.

Our entire international economy seems to boil down to a matter of the cost of nitrogen, because in our foods we determine the ultimate cost by calculating the amount of protein present – the more protein, the higher the cost of the food.

Starving countries live on low cost carbohydrate diets, their soil is nitrogen deficient, their crops are therefore poor, and all this on a planet with an atmosphere made up of almost 80 percent nitrogen.


Phosphorus is sometimes found in plant proteins and it is an important building block in the nucleus of every living cell.

Certain phosphorus compounds are responsible for the transfer of energy.

When you eat a candy bar and then spade an extra ten rows in the vegetable patch, phosphorus compounds transfer the sugar energy to your working muscles.

Insufficient phosphorus results in stunted plants and abnormal cell division.

Phosphorus-hungry plants may be an abnormal dark green and the leaves may be held at a peculiar, forward-pointing angle.

The phosphorus in fertilizers may be in any of several forms, some of which are rock phosphate, the several superphosphates, ammonium phosphate, bone meal, bone ash, tankage, guano, basic slag and the naturally-occurring mineral called apatite.


Potassium has been a problem for plant physiologists and soil chemists.

Its role in plant growth must have to do with the regulation of various steps in the physiology of the plant.

Potassium is peculiar in that, though it is required to be present in fairly large amounts, it does not appear to be as a part of the plant protoplasm or other structural material.

Experts have found that photosynthesis, food transfer, protein syntheses, cell division and translocation of certain forms of nitrogen within the plant all depend on an ample supply of potassium.

When plants lack potassium a whole complex of symptoms indicate the deficiency.

Stems are weak, leaves tend to curl downward, to bleach at the tips, along the margins and between the veins, dead patches may appear on the leaves and stems and leaves may be discolored, particularly showing purplish striping.

The entire plant will be stunted and seeds will be shriveled.

In fertilizers potassium is always added as an inorganic element, frequently as a mined mineral.

The term “potash” covers the many crude oxides of potassium or, in analysis, refers to potassium oxide.

Common naturally occurring forms of potassium are potassium chloride, potassium sulfate and its derived forms in combination with magnesium, aluminum and calcium, potassium aluminum silicate, potassium iron silicates, and potassium nitrate.

Two Other Major Minor Mineral Elements

At least two other mineral elements find themselves on the borderline between the so-called major and minor mineral elements.

Originally the distinction was made on the basis of the amount of an element that had to be added to a field in order to have a good healthy crop.

Now scientists tell us that at least two of the elements that previously were thought to be essential in only small amounts now are known to be required in fairly substantial amounts.

This whole business of “major” and “minor” is relative – nitrogen, phosphorus and potassium are needed by all plants in “wholesale” amounts.

Others are necessary in smaller quantities, and calcium and iron seem to be next on the list.


Calcium is a structural material in plants. When cells divide and the new walls are formed an early step in the process includes the formation of calcium pectate.

Calcium compounds also are known to affect the water absorbing ability of some cell substances and the water permeability of others.

Calcium is used also by plant cells to neutralize certain waste products of the cell and to prevent the accumulation of toxic plant acids.

When a plant does not contain sufficient calcium it almost always has a poor root system, it is puny, weak and stunted, and these conditions worsen at the tips of the plant.

Calcium deficiency in plants is remarkably similar to rickets (calcium deficiency) in man and animals.

Calcium may be found in some fertilizers in the form of calcium phosphate and related minerals, but, since most calcium compounds are relatively insoluble, calcium is generally added to the soil as a separate dressing of hydrated lime, ground limestone or gypsum.

Such applications must be calculated carefully because lime also affects soil structure. Gardeners frequently use bone meal in order to give a high calcium fertilizer to their favorite lime loving perennials.


Iron is the other in-between essential element.

Plants frequently contain remarkable amounts of iron, using it in the synthesis of chlorophyll, though it is not found in the chlorophyll molecule, and in plant respiration.

Since iron is essential to chlorophyll formation, iron-deficient plants are usually yellowish and stunted. If iron is very deficient, plants will be completely dwarfed and yellow.

The story of iron availability is a complex one. In alkaline soils, iron, even when present in high amounts, is unavailable to most plants.

By adding humus to soil the naturally occurring iron is made more available. Iron may be found in fertilizers in an inorganic form such as iron sulfate or iron chloride (ferric sulfate or ferric chloride).

Blood meal contains quite a bit of iron.

Recommended Reading

  • Fertilizer with Iron
  • Potassium in Plants
  • Calcium in Plants
  • Phosphorus in Fertilizer

By John Baumgardt