PLANT NUTRIENTS AND THEIR ABSORPTION 129 



been affected by previous plant associations that have grown on it. 

 Humus is therefore more abundant and the plants are adapted to soils 

 of relatively high acidity and great nitrogen content. Hence, lime 

 is most useful for crops and nitrogen the fertilizer most often required by 

 fruit trees. It is more profitable to grow cereal crops on the great plains, 

 prairies, savannahs and pampas while fruit trees thrive best in the regions 

 of coniferous and deciduous forests. 



Summary. — The various mineral elements and nitrogen are absorbed 

 by the plant from the soil solution. These mineral elements, except a 

 portion of the sulphur, may be recovered in the ash of the plant. In 

 addition to the necessary mineral elements, the ash generally includes 

 small quantities of a number of non-essential elements occurring in the 

 soil solution. The ash content of plants varies with the kind of plant 

 and with the soil upon which it is grown. The ash content of different 

 tissues also varies with the kind of tissue, its age and the season. Nutri- 

 ent elements must not only be in solution but must be in an available 

 form — that is, combined with certain other elements and in certain 

 compounds. Nitrogen is absorbed mainly as nitrates. The nitrate 

 supply in the soil is subject to great fluctuations, depending on tem- 

 perature, moisture, aeration, bacterial activity, the supply of nitrogen- 

 carrying materials from which nitrates can be formed and many other 

 factors. An important part of the orchard soil fertility question consists 

 in maintaining a liberal supply of nitrates in the soil during the growing 

 season. Most crop plants prefer a soil practically neutral in reaction. 

 Deciduous fruits are distinctly acid tolerant and certain of them thrive 

 best in an acid soil. The best orchard cover crops are likewise acid 

 tolerant. The chlorotic conditions frequently found in strongly cal- 

 careous and manganiferous soils apparently are due to iron starvation 

 incident to an alkaline reaction. Many organic disintegration products 

 are known to be toxic to certain crop plants and there is evidence that 

 they are often of considerable importance in determining the produc- 

 tivity of orchard soils. Some of the injurious effects of sod upon trees 

 evidently are due to these toxins in the grass land. Excessive concen- 

 trations of certain salts, particularly of sodium and potassium, are toxic to 

 orchard trees and give rise to the so-called "alkali" conditions. Treat- 

 ment for disorders of this kind may be both remedial and preventive. 

 Optimum conditions for absorption are provided when the various 

 nutrient elements are found in the soil solution in certain rather definite 

 proportions. Sometimes harmful influences result when these ratios 

 do not obtain. Both transpiration and soil aeration influence somewhat 

 the rate of absorption. Within certain limits plants are able to absorb 

 from the soil solution the elements most necessary, taking them out in 

 proportions sometimes very different from those in which they exist. 



