414. SECTIONAL TRANSACTIONS.— M. 



supply of nutrients up to a certain point. The relationship is not simple : it can be 

 expressed by two factors, one being the minimal amount in 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 the 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 efiective 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 

 nutrients, and conversely a good nutrient supply economises water. For fruit 

 the relationships are somewhat different, fruiting and growth being in some ways 

 antagonistic. These relationships are important in irrigation practice. 



The second way in which nutrients affect the plant is 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 is such as to give a 

 normal plant, variations in the total amounts have but little effect on composition 

 or habit of growth ; the individual plants may be larger or smaller, but the material 

 of the plant is not much affected. As soon, however, as the proportion of any one 

 element falls too low, certain characteristic effects are produced on the plant which 

 may profoundly alter its reaction to external conditions, and its chemical com- 

 position, and therefore its agricultural value. 



In certain conditions any of the elements may thus be in deficiency. Considerable 

 investigation has 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 produces 

 characteristic efiects, which are being studied. 



In humid regions deficiency of nitrogen relative to the other elements is common, 

 the result of the ready solubiUty of the soil nitrates. 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 Unked with the organic matter 

 content and the microbiological activity of the soil. In the writer's experience it does 

 not occur in dry regions. 



Phosphate deficiency may arise anywhere, but especiaOy on 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 innutritious to 

 animals, inducing phosphate deficiency diseases which have been much studied by 

 Theiler, du Toit and Green in South Africa. 



Iron deficiency is perhaps more common than is usually recognised. B. C. Aston 

 first found it in certain New Zealand soils and traced to it 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 recognisable 

 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 the fertilising effect of sulphur in these conditions may arise from an effect 

 on the iron compounds in the soU. 



To manganese deficiency has been attributed a disease of oats, and to magnesium 

 deficiency a chlorosis of tobacco. 



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 the percentage of nitrogen compounds correspondingly increases. The grain 

 of barley suffers in malting quality, the tubers of potatoes suffer in cooking quaHty, 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 the 

 exchangeable bases in the soil, and with the whole body of its physical and chemical 

 properties. It is most liable to occur in wet regions, where it results in an acid soil. 

 It 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 the properties of the soil are drastically altered, 



