CHAPTER XXII 



HOW THE NEEDS OF SOILS MAY BE DETERMINED 



Chemical Analysis Has Limitations. — ^At first thought it would 

 seem that since the requirements of the different crops are known, 

 the f ertihzer needs of any particular soil can be determined simply 

 by a chemical analysis of that soil. This, however, is not the case, 

 because several factors must be considered in addition to the 

 quantity of the plant-food elements contained in the soil — such 

 as, the plant characteristics, the kind of soil, availabihty of the 

 elements, and all the other factors which determine fertihty. 



What might be the fertihzer needs of a particular soil which 

 analyzes 0.3 per cent nitrogen, 0.08 per cent phosphorus, 1.3 per 

 cent potassium, and is not acid? ^ No one can tell with any degree 

 of certainty without considering carefully all the factors named 

 and to which reference is made (p. 80). It is evident, therefore, 

 that the chemical analysis of a soil has certain limitations. It is 

 not to be understood, however, that chemical soil analyses are of 

 no value. Because of these limitations, other methods than 

 determining the total amounts of nitrogen, phosphorus, and potas- 

 sium in soils have been devised to ascertain the fertilizer needs 

 or the cropping possibilities of soils; such as, availability tests, 

 pot tests made in the greenliouse, cylinder tests made indoors 

 or out of doors, and fertilizer tests made on the farm uiider 

 field conditions. 



Other Factors First. — ^Before deciding upon a chemical analysis 

 to solve a soil fertihty problem, the following factors should 

 be considered: 



(a) Drainage. — ^Very often crop yields are low, or some crop 

 fails entirely, because of the lack of proper imderdrainage, even 

 though the soil may be rich in all the elements. This factor should 

 be considered on sloping fields, on hillsides and at the foot of 

 slopes or hills as well as on lowlands (Fig. 46, Chapter IX). 



(b) Moisture Supply. — Frequently the lack of sufficient mois- 

 ture, even in humid climates, is the cause of low yields. This 

 lack of moisture may be the result of insufficient rainfall during 



^ This particular soil responded well to acid phosphate and potash in 

 the field. 



355 



