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Looking
After
Your
Soil
By
Jack Cox
The only way to really understand the soil is to get your hands dirty, but it is important for any serious smallholder to study at least a little of the subject. So in this series of articles I shall be looking at some of the theoretical issues involved. This is at best a brief introduction to a very complex subject. I have tried to avoid being too dry and academic but only you can say whether or not I have succeeded.
The soil is the most important single factor on our fincas. It is the basis of our success, and ultimately of our very survival. Our soil took many thousands of years to form but it can be utterly ruined by only a few years of bad management.
For most practical purposes we can consider soil fertility as being the capacity to produce healthy plants; however each crop is fairly specialised for a particular type of soil and an unhealthy plant is not necessarily an indication of a poor soil. Some people treat their soil as if it were just an inert geological substance but it is far more than that, it is our most valuable asset. All forms of terrestrial life, including ourselves, depend on it for their very existence. The soil is in fact an entire community of mineral substances and living organisms which together make up a complete miniature ecosystem.
There are six major constituents of living soil. These are: mineral particles, humus, micro-organisms, small visible organisms (such as earthworms), air and water. It is often useful to also consider the plants and animals which live in and on the soil to be part of the soil ecosystem. This is because the soil cannot endure for long without plants; and plants can not survive indefinitely without animal by-products. In nature such an imbalance is not possible.
None of these soil constituents are individually significant because fertility is largely the result of interactions between all of them. The balance of these constituents varies with depth. This can be seen by taking a vertical cut through the soil, thereby exposing the soil profile. We often refer to top soil and subsoil, although most soils have at least four levels known as “horizons”.
MINERAL CONTENT OF THE
SOIL
Most of the mineral particles in the soil are derived from the parent material (rock). However, minerals may also be formed within the soil itself. Weathering releases elements from the minerals already present and these pass into the soil solution. Only nutrient elements in solution can be taken up by the plants.
Minerals may be present in a range of particle sizes. They vary from minute clay particles (less than 0.002 mm. diameter) to the relatively large particles of sand (0.05 to 2.00 mm. diameter). Between these two extremes are the particles of silt (0.002 to 0.05 mm.) It is upon the mixture of these “fine earth” particles that the texture of the soil depends. A clay soil has very small pore spaces which tend to retain moisture. If clay soils are worked when wet they will puddle, which may be great for potters but it is the curse of many growers. Although clay soils are often rich in nutrients, they are heavy to work and slow to warm up in the Spring. This makes them unpopular with most horticulturists.
On the other hand a soil which is dominated by sand will have large pore spaces and is likely to be free draining. This is likely to be the case on most of our fincas. These sandy soils are equally unpopular because they tend to be less fertile, and do not retain water during dry weather; a big problem in the drought conditions of a Spanish summer! These are said to be hungry soils because they take as much manure or compost as you can give them without showing much improvement.
If these various particle sizes are present in roughly equal proportions the soil is said to be loamy. This is generally the best type for horticultural purposes. Of course such land is highly desirable making it expensive to buy. You pay your money and you make your choice!
SOIL NUTRIENTS
The three major
soil nutrients are Nitrogen (N), Phosphorus (P) and Potassium (K). As these are needed by crops in fairly large quantities, they are called macro nutrients.
If you buy chemical fertilisers, its NPK ratios will be printed on the bag. From this you may well assume that the bag you buy is all your plants need. Not so.
Plants are only able to take up a tiny proportion of the inorganic fertilisers you add to the soil. Most of it get leached out of the soil into he ground water. And from there it is washed into your nearest river to kill the fish and poison your drinking water.
There is another product which is much better than NPK. This product delivers all the nutrients the plants need in a form they can readily use. This product is called FYM - farm yard manure!
Now
lets take a closer look at the three major nutrients.
OIL NUTRIENTS - NITROGEN
Soil minerals contain several essential plant nutrients. Of these the major three are nitrogen (N), phosphorus (P) and potassium (K). These are called macro nutrients because they are needed in fairly large quantities. Plants also need many trace elements such as calcium, magnesium, iron, sulphur, manganese, boron, molybdenum, copper and zinc.
Nitrogen is normally assimilated by plants in the form of nitrate, although under certain circumstances,
ammonium salts may be utilised directly. Although nitrogen is a major constituent of the air, plants are unable to obtain it from this source. They generally take up nitrates from the soil solution via their roots. Nitrogen is extremely important for the growth of green leaves but has little effect upon the development of roots. This is why brassicas are able to benefit from recently manured ground while root crops can’t. In fact, too much nitrogen is a disadvantage for root crops as most of the nutrient goes to produce bushy tops at the expense of the edible portions.
Nitrogen deficiency is easily detected by a characteristic yellowing of the leaves and stunted growth. levels of nitrogen can vary widely between different soil types: Soils rich in organic matter may contain 1% or more (by volume) of nitrogen while some light sandy soils have less than 0.05%. Sensibly managed arable land is unlikely to contain less than 0.1%. Unfortunately, these figures shed little light on the amount of nitrogen which is actually available to the plants. As I said earlier, plants take up most of their nitrogen in the form of dissolved nitrates. Nitrogen is converted to nitrates by the action of micro-organisms, particularly bacteria. So, in order for plants to benefit from the nitrogen, the soil must be in good heart. It must contain an abundant population of micro-organisms. Where large amounts of chemical fertilisers are applied the action of the micro-organisms can be impaired and the levels of nitrates available to the plants can actually decrease.
The chief natural source of soil nitrogen is plant and animal residue. The ratio of nitrogen to carbon in plant remains, lies in the range of about 1:40 to 1:25. The latter figure refers to legumes. The ratio of nitrogen to carbon in humus is about 1:10. Certain bacteria in the soil are capable of fixing atmospheric nitrogen. Another group of bacteria live in close association with the roots of legumes (peas, beans etc). This is continuously being leached out of the soil by rainwater. This is a particularly serious problem where land is left fallow. For this reason, it is usually preferable to have something growing in the soil at all times. Even when the land is not needed for a food crop, it can be used to grow “green manures” - a crop dug into the ground in the Spring. this also has the advantage of suppressing weed growth, thus preventing soil erosion and supporting the soil ecology.
SOIL NUTRIENTS -
PHOSPHOROUS
PHOSPHOROUS is another important plant nutrient. It is a constituent element of many of the compounds which make up plant tissue. Phosphorus also plays an important part in the growth and development of plants and in the production of carbohydrates.
Phosphorous is naturally present in most soils, although it is not always released to the plants in sufficient quantities. Plants take up phosphorous in the form of phosphates.
A phosphorus deficiency may show up as a purple decolouration in seedlings, followed by yellowing as the plants get older, stunted growth and lateness in coming to maturity.
Phosphorous may be added to the soil in several forms. The most common is known as BASIC SLAG. This is a byproduct of the steel making industry, being the ground up limestone lining of blast furnaces.
The word “basic” in this context is synonymous with “alkaline” so it will also raise the pH of your soil thus correcting acidity.
In recent times, new steel making processes have led to basic slag becoming harder to get.
A good alternative is GROUND ROCK SULPHATE. This has the advantage of being released more slowly, over a longer period of time, in the way preferred by plants (and by organic growers)
SUPER PHOSPHATE is made by dissolving rock phosphate (and sometimes animal bones) in sulphuric acid. However this product is rather expensive to buy and, being strong and quick acting, needs to be used with care if it is not to harm your soil organisms. Most organic growers avoid it like the plague!
It is usually considered to be a waste of resources to put phosphorus into general purpose fertilisers. This is because plants are only able to use a small amount (often less than 15%) of the added nutrient.
Most of the plant’s phosphorus intake is in the form of organic compounds. Humus is a very rich source of these compounds and phosphorus is unlikely to be a problem in a healthy, humus rich soil.
SOIL NUTRIENTS -
POTASSIUM
POTASSIUM (K) is another major nutrient, essential to plant growth. It is needed in quite large amounts.
Potassium exists in solution within the plant where it assists in many metabolic processes.
It aids the production of proteins and increases the vigour of the plant; promoting disease resistance and producing strong stems and stalks.
Potassium improves the quality of fruits and tubers as well as the viability of seeds.
Most soils contain ample amounts of potassium. However, it is only when it is in a solution, in the form of organic compounds, that it is actually available to the plants.
Potassium deficiency may show itself by a yellowing and withering of the leaves and the production of thin, weak stems. If your plants fall down in heavy rain or strong winds, test for potassium.
Other signs are that your fruit may lack colour and your potatoes may turn black when cooked.
Light sandy soils are more likely to be a problem than are heavy clays.
There are huge natural deposits of rock-potash in many part of the word. As long as these last, potassium deficiency in soils remains a simple matter to correct.
Rock potash contains about 10% potash and is very long lasting. One application may last up to three years. About 225 grams per
m2 is usually about right.
HUMUS
HUMUS is a collection of microscopic colloidal particles, formed by the decaying of organic matter. It is further broken down within the soil by various micro-organism to form the foodstuffs which plants can use.
In heavy soils, drainage and aeration are a constant headache. Humus bonds chemically with the clay particles to form CLAY / HUMUS COMPLEXES.
This process forms particles which are considerably larger than the independent clay ones and so the pore spaces in the soil are correspondingly enlarged.
So humus added to clay soils will make them lighter, easier to work and generally more productive.
When added to the light sandy soils that most of us in Spain have, the humus improves the soil texture as well as increasing water retention and the organic content.
It is hardly surprising that humus is rightly regarded as a panacea for all soils.
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