THE INFLUENCE OF FERTILIZATION ON THE QUALITY OF SPINACH AT VARIOUS LIGHT INTENSITIES
As oxalate, and especially free oxalic acid, are undesirable products of plant physiological processes it is necessary to know the factors causing oxalate production in the plant. Both the increasing influence of calcium, nitrochalk and potassium and the decreasing influence of phosphate and ammonium salt are well-known. The interaction between light intensity and oxalate production in spinach is affirmed by a great number of authors. Most of them mention the stimulating influence of light without giving specific results.
We therefore undertook a number of investigations to ascertain whether the light intensity had a stimulating effect or not. Four sowings a year were grown in the field on a sandy loam for several years: one in the early spring, two in the summer and one in the autumn.
Treatments were as follows:
Treatment 1 without nitrogen
Treatment 2 40 kg N/ha
Treatment 3 80 kg N/ha supplied before sowing as nitrochalk
Treatment 4 120 kg N/ha
Treatment 5 + 6 received 40 kg N/ha for initial fertilization and were made up to 80 and 120 kg N/ha respectively later during the growing period. All treatments were fed with 60 kg phosphoric acid per ha as Thomas meal and 100 kg potassium per ha as Patent potassium. Six samplings per sowing were investigated. The first sampling was done at the four leaf stage, and the last at the marketable stage of the spinach.
During growth, spinach shows a decreasing oxalate and oxalic acid content in relation to the dry matter; but in relation to the fresh material the reverse is true. The oxalate and oxalic acid content of the dry matter increases as the amount of nitrogen supplied to the plant increases if climatic conditions do not adversely affect nitrate reduction.
As it is not possible to quote all results within the time available, only six test series were selected out of a great number. Two of these series were grown in the spring, two during summertime, and two in the early autumn so that the results are spread over a broad range of climatic conditions.
The yield of marketable spinach ranged from 0.6 to 1.2 kg/m2 for the treatment without nitrogen fertilization and from 1.5 to 3.8 kg/m2 at the highest rate of nitrogen supplied in one dose before sowing. Dry matter content of this material varied from 6 to 14% according to climatic conditions.
To demonstrate the results some graphs from two sowings in the summer show the production of fresh material, dry matter content and the oxalate, free oxalic acid, total nitrogen and nitrate content of the dry material.
The first sampling was done 20 days after sowing, and the sampling at the marketable stage was carried out 39 days after sowing. In this test series, grown under relatively dry conditions, the yield per unit area increases with enhanced nitrogen dose. The treatments with nitrogen supplied in separate doses do not yield as well as those receiving the same amount