SOIL FERTILITY.

New Zealand Herald, Volume LXVI, Issue 20420, 23 November 1929, Page 22

SOIL FERTILITY.

QUESTION OF CONTROL.

EFFECTS OF DEFICIENCIES.

SIR JOHN RUSSELL'S VIEWS. The subject of soil fertility and its control was exhaustively discussed by the agricultural section of the British Association at the recent meeting in South Africa. Sir John Russell, director of Rothainsted Experimental Station, who led the discussion, defined a fertile soil as one which satisfied all the conditions of plant growth, adequately supplying plant nutrients, water, warmth, air and space for the roots and which was free from undesirable substances or harmful reaction.

" The supply of plant nutrients affects crop production in two ways," said Sir John. " Oilier conditions being favourable, the amount of plant growth is increased with increasing supply ot nutrients up to a certain point. Tho relationship is not simple: it can be expressed by two factors, one being t lie minimal amount iu the crop of the particular nutrient, and the other the supply of the nutrient already present in the unmanured soil. Some degree of proportionality between tho various nutrients is necessary, but there is no evidence that the ratios are narrow. These relationships are much affected by the water supply. In general, nutrients are most effective when there is a good water supply, and the water is most effective when there is a good nutrient supply. A good water supply thus economises the nutrient, and conversely a good nutrient supply economises water. For fruit the relationships arc somewhat different, fruiting and growth being in some ways antagonistic. These relationships are important in irrigation practice." Proportion Between Nutrients.

The second way in which nutrients affected the plant was to change its composition, habit of growth, and response to external conditions, including the attacks of insect and fungus pests. So long as the proportion between the different nutrients was such as to give a normal plant, variations in (he total amounts had but little effect on composition ox habit of growth; the individual plants might be larger or smaller, but the material of the plant was not much affected. As soon, however, as the proportion of any one element fell too low, certain characteristic effects were produced 011 the plant, which might profoundly alter its reaction to external conditions, and its chemical composition, and therefore its agricultural value. In certain conditions any of the elements might thus be in deficiency. Considerable investigation had been made to discover the effects of these deficiencies, and also their symptoms, so that the agricultural expert might be in a position to recognise them. Large excess of certain of the elements relative to the others also produced characteristic effects, which were being studied. " In humid regions deficiency of nitrogen relative to the other elements is common, the result of the ready solubility of the soil nitrates," continued Sir John. " This reduces the rate of growth and the total amount of growth, but otherwise has little effect on the composition or character of the plant. Nitrogen deficiency is closely linked with the organic matter content and the microbiological activity of the soil. In my experience, it does not occur in dry regions. " Phosphate deficiency may arise anywhere, but especially oil soils derived from rocks containing little or no animal remains. It reduces the root activity of plants and the tillering of cereals, and brings about certain chemical changes profoundly affecting the quality of the. produce. Phosphate starved grass is innutritions to animals, inducing phosphate deficiency diseases which have been much studied by Theiler, du Toit, and Green in South Africa.

Deficiency of Iron. " Jron deficiency is, perhaps, moro common than is"usually recognised. JJ. C. Aston first found it in certain New Zealand soils, and traced it to a persistent anaemia of cattle, from which.they suffered greatly, and finally died. This was in a wet region on somewhat acid soil, sufficiently light to allow of ready leaching out of the iron from the surface layer. A similar disease is said to occur elsewhere in similar conditions, and may have the same cause. Unfortunately, no easily recognisabb symptoms in the vegetation have been observed, but analysis of the ash at once reveals the deficiency of iron. A like deficiency may occur in dry regions: possibly some of (he fertilising effect of sulphur in these conditions may arise from an effect on the iron componds in the soil " Sir John pointed 011 L that potassium deficiency is usually found only in relation to nitrogen excess. ".When the ratio of nitrogen to potassium becomes large, the plant is considerably altered; its leaves become very dark green, liable to die in discoloured patches, liable also to attack by certain fungi; the percentage of starch or sugar in the storage organs falls off, and thd percentage of nitrogen compounds correspondingly increases. The grain of barley suffers in malting quality, the tubers of potatoes suffer in cooking quality, and the roots of sugar beet not only contain less sugar, but yield a more impure juice. ' " Calcium deficiency is in a different category, being closely linked up with I the exchangeable bases in the soil, and l with the whole body of its physical and j chemical properties," he added. "It . is most liable to occur in wet. regions, | where it results in an acid soil. Jt j may occur in dry regions, however, especially where sodium chloride is present. and the sodium has displaced some of the calcium. When this replacement has proceeded beyond a certain stage, (ho properties of the soil are drastically altered, making it unsuitable for many agricultural crops. This change is of great importance in irrigation iiieas, not! it is an important factor in the evil effects of overwatering."