2.2 The Functions and Qualities of the Soil

The functions of the soil in agriculture are: supporting plants; holding and supplying nutrients, water, and air to plants; and, providing a healthy condition for plants to grow. Good soil fulfills all three of these functions very well. Then what kind of soil is good? The farmer's perception of good soil is usually that it is rather black in color, soft, and rich in micro-organisms and earthworms, etc. In technical terms this translates into: well structured; optimal moisture: rich in nutrients; and, high in biological activity.

The qualities of good soil can be divided into physical, chemical and biological. Really good soil is well balanced and of high quality in these three areas.

2.2.1 Optimal Physical Qualities

Soil should have high water-holding capacity but also have good drainage. Soil which is considered to be of good physical quality or be well structured carries out both functions well.

Soil is mainly made up of three types of matter: solid (minerals and humus), water and air. Whether or not soil is well structured depends on the allocation of each of these components. If there is too much solid, the soil becomes hard. The soil should be soft enough for plant roots to be able to penetrate. Too much water in soil reduces the percentage of air and results in oxygen deficiency for plants roots. Too much air in soil causes drought. Thus an optimum allocation of water, very important in determining a good soil.

The allocation of well structured soil is usually 40% solid (out of this, humus is 5%), 30% water, and 30% air.

The type of soil is determined by the amount of these different components. Clay soil is high in water holding capacity but low in air supply. Sandy soil is high in air content and supply buy low in water holding capacity. The difference between clay and sand is the size of pore spaces are those which can gold both water and air equally. Clay has small particles and small pore spaces so that if water comes the pore spaces are filled with water and air is forced out. Sand has big particles and big pore spaces so that though water comes in, air infiltrates and the water is forced out. Therefore, clay and sand mixed soil is appropriate for agricultural land.

Though the type of soil may be the same, some soil is well structured and other is not. Or, in other words, the same soil can be good or bad. The reason for this is the amount of humus in soil. Because of the characteristics of humus, soil which is rich in humus (more than 5%) is very well structured. First, humus is like a paste which combines small soil particles and makes a crumble structure (optimum size of particles and pore spaces). Second, humus is very high in water holding capacity. As a result of these characteristics, if enough humus is supplied, clay soil becomes good in drainage and sandy soil becomes good in water holding capacity. It is very important to note that it is only humus which improves soil structure effectively.

We have learned from the nutrient cycle that humus is made from organic matter by micro-organisms in the decomposition process and disappears in mineralization. Humus doesn't stay forever in the soil. Thus if the supply of organic matter is stopped, the structure of the soil degrades. Chemical fertilizer never develops the structure of the soil. Rather, it destroys it by killing micro-organisms and accelerating mineralization. The main reason for soil structure degradation in Bangladesh is over dependency on chemical fertilizer and lack of organic matter (humus) supplied to the soil.

2.2.2 Optimal Chemical Qualities

The chemical qualities of the soil are the functions which are supported by chemical action. Soil which is chemical of good quality supported by chemical action. Soil which is chemical of good quality usually has a high nutrient-holding capacity and optimum soil PH.

Nutrient-holding Capacity (CEC)
When minerals dissolve into water, they are divided into cation and anion through chemical action. Most nutrients (minerals) necessary for plants are held in the soil in the form of cations with the colloids except for a few such as phosphorous. Plant roots take these minerals by exchanging cations with the colloid. Therefore, the degree of CEC (Cation Exchange Capacity) of the soil is used by soil scientists as an indicator of the nutrient-holding capacity of the soil.

Whether or not soil has high CEC, depends on the quality and quantity of colloids in the soil. Good quality colloids can keep many cations while poor quality colloids cannot. Colloids are supplied by clay and humus. Sand has no colloids. Therefore, sandy soil is very poor in nutrient-holding capacity while clay soil is much better. The best colloids come from humus. The humus colloid is the deciding factor in whether or not soil becomes good in nutrient-holding capacity of the soil.

Almost all farmers who use chemical fertilizer complain that they must increase the quantity of chemical fertilizer each year, otherwise they cannot maintain yields at the same level. The reason for this problem is the degradation of the nutrient-holding capacity. Dependency on chemical fertilizer for fertilization and lack of organic matter supply reduces the quantity of humus and humus colloids in the soil. Consequently, it becomes poor in nutrient-holding capacity and farmers need more and more chemical fertilizer to compensate. Chemical fertilizer increases the availability of the main minerals (N.P.K.) but it never develops the CEC.

Soil PH
Soil PH indicates whether oil is acidic, neutral or alkaline. Levels go from 1 - 14 with 7 neutral. A smaller number than 7 indicates acidity and a larger number indicates alkali. Plants cannot grow or absorb some minerals in soil which is too acid or alkaline. The optimum PH level for plants is 5.5 - 7.5. Keeping and regulating soil near neutral PH 7 is a very important part of agricultural practice.

Humus has the important function of regulating soil PH. Humus itself is neutral and it can absorb acid and alkali shock from outside. Application of chemical fertilizer makes soil acidic as it is acidic in nature and has no function to regulate PH. Application of calcium is recommended for neutralization of the soil acidity by scientist but this is only a temporary solution and creates other problems (Section 3.1.1) .

2.2.3 Optical Biological Qualities

The biological characteristics of the soil are the functions which are supported through he activity of micro-organisms in the soil (more than 100,000,000 numbers in 1 gram of fertile soil). Their activities and balance are the deciding point of whether the soil is of good biological quality or not.

Decomposition and Mineralization
As we learned from the nutrient cycle, micro-organisms have an important role in forming soil and supplying nutrients to plants by producing humus in the decomposition process and releasing minerals in the mineralization process. Decomposition and mineralization are indispensable for soil and plants. The more active micro-organisms, the humus and minerals are available for soil and plants. Therefore, the supply of organic matter, which is food for micro-organisms, is a must for soil improvement -physically and chemically. Unfortunately, farmers do not put emphasize on supplying organic matter to soil nowadays because of chemical fertilizers.

Soil Health
Another important role of micro-organisms is to promote soil health. There are some micro-organisms (nematodes, fungi, virus, etc.) which are the cause of plant diseases, but the numbers of these disease-causing micro-organisms are very few compared with other harmless and beneficial micro-organisms. If the balance of micro-organisms is not disturbed, the disease-causing micro-organisms are controlled at levels that do no harm to plants.

For example, there are more than 200,000 kinds of nematodes. Out of these, only 2% are known to be harmful to plants and the other 98% are harmless. As well, within the 98% are some which actually minimize the harmful nematodes. Some algae also eat harmful nematodes. The nematode problem never occurs in conditions where micro-organisms are balanced. Nearly 90% of plants diseases are caused by fungi, however some fungi produce disease curing matter (penicillin from blue fungus etc.). In well balanced soils, the number of fungi are low in comparison with bacteria. This results in a high B/F (bacteria/fungi) ratio.

A situation where micro-organisms are well balanced in created by sufficient quantity and types of organic matter supply and the elimination of disturbing factors. Chemical agricultural practice disturbs through a lack of organic matter supply, addition of agricultural chemicals and an anti-natural cropping system. (Chapter 6)