12 Mineral Nutrition of Plants 



of irrigated soils are often further complicated by excesses of soluble 

 salts, of salts in general, or of some specific toxic salt like borax. 



Here is an important field where soil science, plant nutrition, and 

 plant breeding come together. A good deal of active research is under- 

 way but more is needed. In view of the high yields that can be obtained 

 with improvements in combinations of practices, we can expect the 

 Desert soils to make significantly larger contributions to the world food 

 supply. 



Latosols * 



Vast areas of potentially arable soils exist in the tropics, and here 

 may rest a great deal of the future hope of mankind for abundant food; 

 but the problems are great, and the research to guide us is little. 



These red soils of the tropics, Latosols, are strongly weathered and 

 highly leached. By standards used in temperate regions, they are low 

 in all plant nutrients. Furthermore, they have low base-exchange capac- 

 ities and high phosphate-fixing capacities. 



The tropics, nevertheless, have several advantages. There is little or 

 no frost. The growing season is as long as the moist season, say six, 

 nine, or even twelve months. The limitations imposed by climate, there- 

 fore, are generally less severe than in the temperate and cold regions. 



Already, we have indications of some peculiarities of management 

 requirements and a hopeful glimpse here and there of production 

 attainable under improved management. 



Important in connection with fertility is the mineral cycle — the up- 

 take of minerals and their return to the soil. Plants in the tropics grow 

 rapidly and decay quickly. If an ion of calcium, let us say, were to cir- 

 culate three times in the tropics to once in the temperate regions; the 

 same ion would theoretically do three times as much work in one place 

 as the other. Equal amounts of plant nutrients, therefore, might go 

 farther in the tropics than in temperate regions. 



Since the Latosols of the tropics have low base-exchange capacities 

 and, hence, low nutrient-holding capacities, a continuous cover of plants 



*A term recently introduced to apply to a broad group of soils in the tropics 

 that are red, leached, relatively high in iron and aluminum as compared with 

 silica, relatively low in base-exchange capacity, and highly aggregated. 



