52 CHAPTER IV 



(e) That the supply of potash is rarely deficient, and that 

 the use of commercial potash fertilisers for common farm crops 

 is seldom profitable. Stable or kraal manure contains a fair 

 amount of potash, so that if much of this material is used for 

 soil improvement considerable quantities of potash will be 

 brought into the soil in an extremely economical form. Old 

 sheep kraal manure is exceptionally rich in potash. Com- 

 mercial forms of potash may frequently be used with profit 

 for a few special crops, such as tobacco and potatoes. 



(/) That the supply of lime as a plant food is practically 

 never deficient, but in the case of " acid " soils applications 

 of lime, in the form of finely-ground limestone, are probably 

 desirable. Before incurring heavy expense by the purchase of 

 large quantities of lime, the farmer is advised to test the effect 

 <of lime on small experimental areas. 



Mixing of Fektilisers. — Most fertiliser materials can be 

 mixed, if necessary, before application to the soil, but the fol- 

 lowing precautions should be observed : — 



(1) Do not mix fertilisers containing all or part of their 

 nitrogen in ammonia form {e.g., sulphate of ammonia, Gov- 

 ernment guano, kraal manure) with lime or fertilisers contain- 

 ing free lime {e.g., basic slag, manure ash, wood ashes). Such 

 mixtures cause volatilisation of ammonia ffas, hence loss of 

 nitrogen . 



(2) Do not mix superphosphate with lime or fertilisers 

 containing free lime, because the latter cause " reversion " 

 of the water-soluble phosphoric oxide to form insoluble in 

 water. 



(3) It is commonly stated that superphosphate should not 

 be mixed with nitrates of soda, potash or lime, and that basic 

 slag should not be mixed with potash salts. 



Application of Fertilisers. — This subject is dealt with 

 in the discussion of the various crops. 



GEEEN-MANUEING. 



Green-manuring is the practice of incorporating green 

 organic matter into the soil by ploughing down a crop grown 

 on the land. Partially decomposed organic matter is known 

 as humus, and the role of humus in the soil is so important 

 {see chapter on Rotations) that the value of any operation 

 which tends to maintain or increase the amount present in 

 the soil can hardly be over-emphasized. 



