SummaryEasily water-soluble substances are classed as instantly available, therefore, not here discussed. Rate of becoming available of substances not easily water-soluble is the important question. How this is affected by some of the variables of the soil, the plant and the climate is indicated. Among these factors are: Physical, chemical, and biological conditions in the soil; the nature of the plant with respect to need for specific nutrients, its length of growing season, its ability to forage for itself; the character of the climate with respect to light, temperature, moisture and the length of the growing season.

Where growing conditions were very similar to those under dry-farming, plants in rows bordering on uncropped areas yielded more than did plants in inner rows. Soil samples taken in planes perpendicular to the crop-fallow boundary lines showed a definite use of soil moisture 6 feet away laterally with sorgo and 4 feet away with corn, and a definite use of nitrates 4 feet away laterally with both crops. Under the main body of the crop corn plants gave evidence of definite absorption of moisture from the ninth foot depth and of nitrates from the eighth foot, while sorgo gave evidence of absorption of moisture from the twelfth foot and of nitrates from the tenth foot. The differences between these crops as disclosed in this study may not be of fundamental nature but may be only differences in degree. It is possible that the conditions as found for sorgo and corn might have been reversed had a much less vigorously growing variety of sorgo and a much more vigorously growing variety of corn been used.

The relation between soil conditions and vegetation, especially of teak forest was studied. Chemical or physical characteristics of the soil only did not give reliable results. Soils were then studied in the field and a detailed soil map of about 1000 ha of teak forest was made. This soil map shows the occurrence and distribution of 29 soil types, which were briefly described. For some soils some chemical and physical characteristics were given in tables. Most soils are Lateritic and Margalitic soils of varying composition and morphology. Often there are horizons with mottled clays or lateritic concretions. Some soils are derived from andesitic, others from quartz-rich or other parent material. The Margalitic soils are developed in heavy-textured clays. Further to this soil investigation an intensive study was made of the vegetation in the same area of Java. The vegetation was classified into various groups according to the development of the forest trees, the shrubs and the ground flora. It was concluded that the soil map was a very important tool for the prediction of the suitability of the land for growing teak.

The relation between soil conditions and vegetation, especially of teak forest was studied. Chemical or physical characteristics of the soil only did not give reliable results. Soils were then studied in the field and a detailed soil map of about 1000 ha of teak forest was made. This soil map shows the occurrence and distribution of 29 soil types, which were briefly described. For some soils some chemical and physical characteristics were given in tables. Most soils are Lateritic and Margalitic soils of varying composition and morphology. Often there are horizons with mottled clays or lateritic concretions. Some soils are derived from andesitic, others from quartz-rich or other parent material. The Margalitic soils are developed in heavy-textured clays. Further to this soil investigation an intensive study was made of the vegetation in the same area of Java. The vegetation was classified into various groups according to the development of the forest trees, the shrubs and the ground flora. It was concluded that the soil map was a very important tool for the prediction of the suitability of the land for growing teak.

It is believed that the spear type of electrode has supplied a long-felt need to the soil specialist and agronomist. In it we have equipment and a method which yields a value that truly represents the pH of the soil under any and all growing conditions. In addition to this it opens up an unlimited field of application to nutritional and plant behavior studies which involve soil reaction in acid, neutral, and alkaline soils. It has been in use in this laboratory for several months and the results may be obtained quickly and accurately and can be closely duplicated. The electrodes are thoroughly cleansed with a stream of distilled water after each determination to remove adhering soil after which the electrodes are gently wiped with soft paper tissue. This leaves the electrodes slightly damp and assures immediate contact with the soil. The Beckman type pH meter was used in these experiments because of the spear type of electrode and the extension cable which especially adapts it to this sort of work.

Field measurements of capillary tension over parts of two growing seasons (1935 and 1936) are presented. Tensions were much higher during the comparatively dry summer of 1936. Relative values of capillary tension at various layers of the soil profile were consistent. Changes occurred where water was being lost or added before corresponding changes reached the other layers. The changes follow closely the precipitation data. Records of the capillary tension for two soils and for three crop relations on the same soil showed decided differences. These differences were maintained over the two summers. Application is made of the same apparatus used in measuring capillary tension for observing variations in the level of the free water table. Experimental curves relating moisture percentage and capillary tensions for samples of the Lordstown surface soil were found to differ, depending on whether the soil was drying or wetting. The drying curve was also changed when the soil structure was disturbed. The use of capillary tension for expressing soil moisture conditions eliminates the uncertainties which are introduced by the hysteresis and structure effects when moisture percentage is used. Capillary tension has the additional advantage of applying equally well for stony soils. Tensions cannot be measured with porous clay apparatus when they exceed 1 atmosphere, but within the range of 1 atmosphere they are readily obtainable.

Investigations were conducted to study various plant responses in Crotalaria juncea to different exposures to light at different stages of development. The efficiency of the plant for green manuring was also determined on the basis of the fertilizing constituents that might be incorporated into the soil at different stages of growth. With an increase in the length of time of light exposure, an increase in the physical characters of the plant was observed, viz., height of the plant, number of leaves per plant, leaf area, root length, number and weight of root nodules, and length and yield of fiber. Some of these characters attained their maximum development under continued illumination, while others required only 20 hours' exposure, longer exposure resulting in solarization. The critical light period for Crotalaria apparently falls within the zone of a 16- to 20-hour day. Beyond this range flowering was completely suppressed and the plant remained purely vegetative. Manurial efficiency increased with an increase in the length of the photoperiod, attaining its highest value under 20 hours' exposure. During the early half of its life cycle, Crotalaria seems to require less light and plants growing under longer exposures are unable to economize the entire energy falling upon them. With further advance in age, however, the light requirements increase. Thus on the basis of these observations, it may be concluded that a photoperiod of 16 to 20 hours considerably increases the manurial efficiency of this crop. If other environmental factors do not interfere, it may be concluded that Crotalaria can be successfully introduced as a green manure crop into regions with long periods of daylight and that it may be grown to advantage in mixtures of early-maturing crops.