How to Determine Fertilizer Requirements of Plants: Methods and Soil Nutrient Value
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Plant fertilizer requirements are determined by two complementary methods: the vegetation method and the field method, which mutually reinforce each other. Used together, they reveal both how much of a nutrient a soil holds and how readily a crop can take it up.
What do the nutritional qualities of soil depend on?
The nutritional qualities of soil cannot be assessed by looking at the soil in isolation from the plant, because what matters is not only the quantity of nutrients present but also their availability to the plant and the plant's response to a given type of fertilizer.
In the experiments of D. N. Pryanishnikov involving phosphorus applied as phosphorite, lupin made excellent use of this source of phosphorus thanks to the acidic reaction of its root secretions, whereas white acacia given the same fertilizer grew no better than it did on soil entirely lacking phosphorus. The example shows that the same fertilizer can be highly effective for one species and useless for another.
How does the vegetation method work?
The vegetation method grows plants in containers known as vegetation vessels under tightly controlled conditions. The soil in each vessel must be thoroughly mixed and uniformly moistened, and the composition and amount of fertilizer applied are set by the aims of the experiment. Each vessel holds the same number of plants, and the vessels are arranged in a vegetation house so that all receive equal light.
On clear days the vessels are wheeled out of the vegetation house on trolleys into a mesh pavilion. The need to apply a particular fertilizer is judged from how the plants grow. If adding phosphorus, for example, does not increase the yield, this indicates that the plant's phosphorus requirement is already satisfied by the soil itself.
How does the field method work?
The field method divides a field into plots of equal size, usually laid out running north to south. Some plots receive a complete fertilizer, while others receive a fertilizer with one element of mineral nutrition deliberately left out, so that the effect of each missing element can be isolated.
Each variant of the experiment must be repeated on several plots, and those plots should be distributed across different parts of the field to account for natural variation. Every plot grows the same number of plants, and the fertilizer requirement is determined from the resulting yield.
What are the advantages and disadvantages of each method?
Each of the two methods has its own strengths and weaknesses. The advantage of the vegetation method is the complete uniformity of the soil and the ability to regulate the water regime; its drawback is the comparatively small volume of soil in which the roots develop.
The strength of the field method is that the root system can develop properly, so data obtained in field trials better reflect the influence of the complex set of conditions actually present in the soil. Its weaknesses are the soil differences that exist even within a small plot and the inability to control water supply and weather conditions.
For these reasons, plant fertilizer requirements are best determined by applying both methods in parallel, letting the controlled precision of the vegetation method offset the real-world realism of the field method.
