Emit Truog 43 



SOILS ARE FRUGAL CUSTODIANS OF PLANT NUTRIENTS 



The question is frequently asked. Is there not great danger of loss of 

 plant nutrients by leaching when fertilizers are added to soils? Or, 

 What about fixation of the nutrient elements by the soil so that plants 

 cannot use them? The answer is that good soils are frugal custodians 

 of the plant nutrients naturally present in or added to soils. If that were 

 not the case, soils would rapidly become so depleted of fertility elements 

 by careless and exhaustive cropping, that a protective covering of crop 

 plants, or even weeds, would not be produced. Under these conditions, 

 erosion would be terrific and disastrous. 



The three availability categories of the nutrient elements 



Just how does a good soil perform the miracle of supplying plants 

 with adequate amounts of nutrient elements and still prevent undue 

 loss by leaching? It does this by forming combinations of the nutrient 

 elements of varying degrees of solubility or availability. For conveni- 

 ence of exposition and consideration, it may be said that nutrient ele- 

 ments are held or stored in soils in three degrees or categories of avail- 

 ability — namely, (a) readily available, (b) moderately available, and 

 (c) slowly or difficultly available. 



To produce a good crop, it is necessary that the supply of readily 

 available forms of nutrients be high enough so that much or most of 

 what is needed by any crop can be obtained directly from these forms. 

 However, it is a great mistake to assume that plants get no nutrients 

 at all from the less available forms. Failure by some to recognize that 

 plants obtain some share of their requirements from nutrient forms of 

 all degrees of availability has caused much confusion in their thinking 

 about this matter. Also, it should be recognized that there is a continual 

 transformation of each nutrient element from forms of one degree of 

 availability to forms of other degrees of availability, and that the main 

 direction of this transformation is determined largely by the relative 

 proportions of the various forms present. 



Figure 4 provides a schematic representation of the three availability 

 categories of plant nutrients in soils. Transformation from one category 

 to the others is represented by arrows in the connections between the 



