Farmers everywhere regularly harvest crops yielding far below the biological potential of their chosen variety.
Although pests and diseases take their toll, and water shortages are widespread, nutritional disorders are probably the most pervasive constraint to crop yields in the tropics.
Most soils used for crop production supply inadequate amount of essential nutrients or contain toxic levels of other elements.
This is the result of either inherent levels low soil fertility, such as that associated with intense weathering in the humid tropic, or of nutrient depletion and organic matter decline caused by repeated cropping without replacing what has been taken from the soil.
In many developing countries such exploitation of the soil through agricultural intensification of this type is being accelerated by population growth and poverty.
Soil fertility problems can be resolved and yield increased by the judicial application of fertilizers, crop residues and /or organic manures. However, such remedies will have a fuzzy element of muck and magic about them, unless the location of specific nutrient requirement of the crop is pinpointed.
In the worst case, farmers may invest in N.P.K fertilizers only to find that they exacerbate the problem by inducing deficiencies of secondary nutrients or macronutrients.
Fortunately, since the German scientist Justin Von Liebig expounded the LAW OF THE MINIMUM in 1840. In modern language it can be stated as follows:
“If several nutrient elements are present in the soil in amounts that would be insufficient for maximum plant yield, the yield will be determined solely by the supply of that element present in smallest amount relative to the plant requirements, variation in the supply of other elements having no effect on yield.”
I.e. as long as P (phosphorus) supply is limiting plant yield, there will be no benefit in supplying increased amounts of any other element. However, once the supply of P (phosphorus) has been improved, another element, in this case N (nitrogen) would be expected to become the limiting element.
Similarly, when additional N has been added, a further limitation is likely to be imposed by the supply of a third element, in this case the micronutrient Zn (zinc). It is clear from this example that although the soil is rather low in the 3 elements N, P & Zn there will be no value in adding N fertilizer until the P deficiency has been corrected. In the same way there will be no benefit in adding Zn fertilizer until both the P and N has been added to the system...
Since then, scientific knowledge of plant nutrition and soil fertility have progressed enormously.
Norman Borlang, Nobel prize winner, commented in 1994. “We believe without doubt that the single most important factor limiting crop yields in developing nations worldwide – and specially among resource poor farmers is soil fertility.”
Soil fertility can be restored effectively by applying the right amount of the right kind of fertilizer, either chemical or organic, or preferable a combination of the two-according to the requirement of different crops, soil types and environments.
SOIL FERTILITY PROBLEMS AND HOW TO RECOGNIZE THEM
The successful correction of soil fertility problems will usually involve 3 steps:
• Recognizing that we have soil fertility problems at a particular site;
• Defining the precise nature of these problems; and
• Finding a cost effective and culturally acceptable solution to these problems.
HOW DO WE KNOW IF WE HAVE A FERTILITY PROBLEM?
Many factors can contribute to affect the fertility of a soil that ultimately will lead to reduced yields. Among them we can mention unfavorable weather (drought or water logging); acid rain; competition by weeds; attack by insect pests; infection by disease organism ; and most importantly shortages or excesses of particular chemical elements in the soil surrounding the roots.
A smart, productivity conscious farmer will help us locating sites with soil fertility problems.
Achtung! In the process of developing village agriculture the best ally in this regard is to have the full cooperation of a dedicated farmer who together with an expert agricultural extension agent can do the following: travel through the area of the village (under or in the process of being planned for cultivation) inspecting crops –under any stage of growing development and make an effort-based on knowledge or past experience –detecting any symptoms that would indicate nutrient deficiencies or excesses.
Since this initial inspection does not require being familiar that could occur on all the crops in the area. It is enough to concentrate on one or two species that are commonly grown throughout the village area, and for which we have good description of the symptoms-e.g. maize, cassava, sweet potatoes or citrus species. (Fortunately, there are useful color illustrated publications that can help us with this task.
Some of these publications deal with all the symptoms likely to be encountered on a particular crop of plant species such as fruit and vegetable in the tropics. If interested in acquiring these publications, contact me.). As we move about the village area, or agricultural sector for that matter, we need to talk to informed farmers and other people who know the local situation well enough to learn all we can about their farming system. We should find out the answer to these questions:
• Are the symptoms we are seeing a common occurrence?
• Are the symptoms present at the beginning or at the end of the cropping cycle? When would we expect nutrient levels in the soil to have been reduced?
• Do they occur in some soils and not in others?
• Are such cultural practices such as including legumes in the system or applying animal manure to the soil?
WHICH CHEMICAL ELEMENTS DO PLANTS NEED?
For healthy plant growth,- we are talking about crops able to yield their maximum biological potential- soils must provide adequate amounts of at least 13 chemical elements These are divided into two main groups: the macronutrients (or major elements), which plants need in relatively large amounts and the micronutrients (or trace elements), which are needed in much smaller amounts.
Major elements or macronutrients: N, P, K, Ca, Mg, S. (nitrogen, phosphorus, potassium, calcium. magnesium, and sulfur)
Trace elements or micronutrients: Cl, Fe, Mn, Cu, Zn, B, Mo. (Chlorine, iron, manganese, copper, zinc, boron, molybdenum).
In addition to the above, there are a few element s which have been found to be needed by some plants or under some circumstances. These are called beneficial elements such as cobalt (which is needed for biological nitrogen fixation), nickel, sodium, and silicon.
HOW MUCH OF EACH NUTRIENT DOES A PARTICULAR CROP NEED?
The amount of each nutrient needed for healthy growth (not just any growth, but a healthy one) i.e. the crop demand for each nutrient depends on two main factors:
• The concentration of each nutrient element needed in the plant tissues for healthy growth and development and
• The amount of plant material that is going to be produced. I.e. harvested for the consumption of man or animal.
Here is a good example of what has been said above. To produce 50 ton/ha of sweet potato you need the amount of nutrients present in tubers plus vine must be N 215, P 38, and K 376.For a yield of only 12 ton/ha the amount of nutrients present in the tuber and vines are; N 52, P 9 and K 90.
That is to say the more you put in (up to a limit, of course) the more you get out--better yield. In order to obtain the maximum biological potential of your crop you have to supply what the crop demand; that is for growth and for production. In this regard the plant is like the cow you feed the cow for growth and for milk production, the need for each stage is different. The more the ration is balanced the more milk production.
HOW CAN WE TELL IF PLANTS ARE GETTING THE RIGHT AMOUNT OF EACH ELEMENT?
There are three important tools that we can use to answer that question:
• visible symptoms of nutrient deficiency or excess;
• plant analysis;
• Soil analysis.
Example when cassava plants are deficient in P the depth of the uppermost branches is reduced from top to bottom by the repeated shedding of the lower (older) leaves. Looking closer the leaves are soft and hang limply from their petioles.
Learning to read deficiency symptoms in the plant is crucial for the farmer. The more you know about the plant needs the more money you get. Remember knowledge is power if used correctly! A well trained eye is all you need. The problem is we only pick up the more severe deficiencies; meanwhile the mild deficiencies will also reduce the yield. Prevention is better than cure. Remember there are books to help you pick up the mild deficiencies!
Chemical analysis of suitable plant parts can tell us a great deal about the plant. Tissue tests are rapid and can be performed in the field (use of test kits) but if N deficiency is suspected test also for P and K deficiency.
The advantage here is that they can give us valuable information before we plant the crop. Now, if we had truly reliable soil chemical tests, for each of the nutrient elements required by plants.
However, the fact remains that there seem to be very few universal soil tests that will work well on all or even most soils.
*Orlando Huaman is an Agronomist and Freelance writer living at Malololelei