436 JOUKNAL OF THE DEPARTMENT OF AGRICULTURE. 



The chemical analysis of the soil is usually designed to furnish 

 the percentages of (a) + (b) ; hardly ever is it attempted to give 

 particulars regarding (b), owing to manipulative difficulties. 

 Frequently, however, an analysis furnishes figures which, on the 

 average, indicate whether there is a sufficiency or not of water- 

 soluble plant food in the soil, i.e. figures showing the percentages of 

 plant food which can be extracted from the soil by means of a weak 

 acid solution, such as a 1 per cent, solution of the acid of lemons, 

 i.e. citric acid. 



The analyses of this report give the figures for (a) + (6), and 

 the percentages of potash and phosphoric oxide extractable by a 1 

 per cent, solution of citric acid, called " available " in the analysis. 



It may be asked with fairness, " Why give the figures for (a) 

 + (?;) at all?" This is hardly the place to discuss the matteii, 

 except to say that the figures for (a) + (b) represent the " capital " 

 of the soil in plant food, and that of this " capital " a very small 

 percentage becomes available year by year. Naturally, the larger the 

 " capital," the greater the output of available plant food to be 

 expected, although the expected does not always happen. 



The output of " available " plant food is governed by the action 

 on the dormant plant food of those several natural processes which 

 are grouped together under the term " weathering." 



Anything that increases the intensity of the weathering processes 

 in the soil increases the output of available plant food. The benefits 

 derivable from thorough tillage are to be attributed in no small degree 

 to the increased " weathering " effect thereby produced. 



Further, by many comparisons of crop yields with the amounts 

 of dormant plant food in the soil, it has been possible to arrive at 

 certain standards of sufficiency and deficiency which, provided other 

 conditions are normal, afford a useful guide. 



Some dormant food substances become available less readily than 

 others, however, a fact which sometimes leads to wrong deductions 

 when the chemical analysis gives figures for (a) + (b) only ; likewise 

 also if the dormant plant food in a soil becomes available at a faster 

 rate than usual. The geological origin of the soil and the figures 

 obtained by extracting the soil with a 1 per cent, solution of citric 

 acid afford a useful check in such cases ; how useful will be apparent 

 when we come to consider the analytical figures for the Paarden- 

 verlies soils. 



It is now necessary to come to closer grips with the actual food 

 substances the plant takes from the soil. Since of the large number 

 required only three (one or more), are ever likely to be present in 

 deficient amount, the discussion will be limited to these three only. 

 They are suitable compounds of phosphorus, of nitrogen, and of 

 potassium. For convenience these compounds figure in the analysis 

 as phosphoric oxide, nitrogen, and potash; it must, hoicever, be 

 reviemhered that it is alioays " compounds " of these substances 

 which are being dealt ivith. 



Nitrogen. — Nitrogen is that food element whose effect is seen 

 particularly in the foliage, etc. AVith a deficient nitiogen supply 

 there is poor, sometimes yellowish, growth ; if the nitrogen supply is 

 adequate the foliage is abundant and of a bright healthy green 

 colour; while, if the nitrogenous supply is excessive, the growth is 

 rank, slow to ripen, does not seed well, and is particularly susceptible 



