20- 



An excess of N increases the need for Ca. In young leaves 

 of rapidly j^rowing shoots most of the Ca may be tied up in the 

 form of calcium oxalate and the supply of this element may tlius 

 be low for other functions. 



Leaf Mg is apt to be suppressed by high leaf K but in fruits 

 there is a positive correlation between K and Mg , showing that an 

 increase in one element is accompanied by an increase in the otlicr. 

 At Ca levels below a certain threshold value, Mg may substitute 

 for Ca . However, increasing Mg from a deficient to an adequate 

 level can actually increase Ca uptake because breakdown of the feeder 

 root system, essential foi- nutrient uptake, is the first effect of Mg 

 de f iciency . 



Roots and Soil 



The growth and function of the roots are closely linked to 

 those of the shoot. The roots are dependent on the shoots for 

 assimilation whereas roots produce hormones and remove from the soil 

 elements essential for shoot activity. An important factor re- 

 stricting root development is the aeration status of the soil. 

 Roots of fruit trees will avoid regions of poor aeration. Thus, 

 on soils with a high water table, the trees will be shallow rooted. 

 Nutrient uptake may be sufficient on these soils, however, shallow- 

 rooted trees will suffer from drought sooner than those with deep 

 roots. A drought will lower the availability of nutrients to the 

 roots, restrict root growth, and may decrease the absorptive 

 capacity of the roots quite markedly. 



Far more insidious conditions occur in soils subject to tempor- 

 ary water logging because of the presence of a hardpan near the soil 

 surface. This occurs on a Wethersfield soil at the Horticultural 

 Research Center where trees will become severely weakened or die 

 for lack of soil oxygen because of temporary water saturation within 

 20 inches of the soil surface in the spring. 



Not all horizons in the soil are equally able to supply nutrients 

 to the tree. The concentrations of most elements are highest at 

 the soil surface and decrease with depth, but the rate of decrease 

 differs between elements. For example, there is a strong vertical 

 difference in K status in soil, K being highest near the surface. 



Under drought conditions the permeability of the roots to water 

 uptake decreases very rapidly; reduction in water permeability 

 reduces the uptake of all ions. In Massachusetts we arc particularly 

 concerned about K and B deficiency and reduced fruit size in drought 

 years . 



