Greenhouse assay of fertility of California soils

This issue reports a new pot-culture technique …

… for studying the supply of available nutrients in California soils. The method gives a fairly reliable indication of whether pasture, field crops, and truck crops on a given soil will respond to nitrogen, phosphorus, or potassium application.

In this technique, various combinations of nutrients (nitrogen, phosphorus, and potassium) are added to pot cultures; Romaine lettuce is grown for definite periods; and relative yields are calculated for partial treatments (N + P, N + K, and P + K) on the basis of yield with full treatment (all three nutrients).

Calibration of pot tests with field tests indicates …

… that nitrogen applications can be expected to increase yields of pasture and field and truck crops if the relative yield of the no-nitrogen treatment in pot tests is 30 per cent or less (provided water, other nutrients, and soil conditions are not limiting).

… that phosphorus applications can be expected to increase yields of these crops if the relative yield of the no-phosphorus treatment is 20 per cent or less (with the same proviso).

Among over 450 California soils tested …

… the percentage found to be low in available nitrogen ranged from 48 to 71 in different soil groups (Storie’s classification).

… the percentage low in available phosphorus ranged from about 40 among alluvial soils (groups I and II) to over 70 among claypans and hardpans (groups IV and V). Both highly acidic and strongly basic soils tended to be low in available phosphorus.

… nearly all appeared to be well supplied with potassium for the crops and soils included in the study.