. Fertilisers and manures. he determining factor whichlimits the yield, or two or more of them may actsuccessively at different periods of the plants poor soils the water-supply is very often the limitingfactor—on very open soils because the water actuallydrains away, on extra close soils because the root rangeis so restricted that the plant has but little water athand and the movements of soil water to renew thesupply are very slow; in either case for comparativelylong periods the plant will be sure to have as muchnutriment as is required for the small growth permittedby the water p
. Fertilisers and manures. he determining factor whichlimits the yield, or two or more of them may actsuccessively at different periods of the plants poor soils the water-supply is very often the limitingfactor—on very open soils because the water actuallydrains away, on extra close soils because the root rangeis so restricted that the plant has but little water athand and the movements of soil water to renew thesupply are very slow; in either case for comparativelylong periods the plant will be sure to have as muchnutriment as is required for the small growth permittedby the water present. It is only when the water-supplyis sufficient that the resources of the soil, as regards allor any of the constituents of a fertiliser, are tested andmay become in their turn the limiting factors in thegrowth of the crop. Hence it follows that fertilisersmay often be wasted on poor land, where growth islimited by the texture of the soil, by the water-supply, or I Yield Jushels or Cwt Returns & Mineral Manures +200 lb. *400lb. - +600lb. +800 lb. 5.—Rel.\tion between Cost of Production and Returns with V.\RYING oUANTITIES OF MANURE. [To face page 284. X.) SOLUBILITY OF FERTILISERS IN SOIL WA TER 285 by some other factor liardly controllable by the farmer :it is a truism that poor land cannot be converted intogood by manuring and that fertilisers give the bestreturns when applied to a good soil. One fundamental difficulty still remains in consider-ing the action of fertilisers ; it has already been pointedout that a soil by no means notably fertile may containenormous quantities of plant food, which is howevercombined in so insoluble a form as to reach the plantin quantities insufficient for the requirements of thecrop. For example, a soil may contain 01 per cent.,or 2500 lb. per acre, of phosjihoric acid, and )et }ield avery indifferent Swede crop unless it be supplied withan additional dressing of 50 lb. per acre of solublephosphoric a
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