Quantitative relationships for growth and N content of different vegetable crops grown with and without ample fertilizer-N on the same soil

Abstract

Unexpected relationships were found to cover various features of N-response of 12 different crops grown in 26 experiments on adjacent sites in the same field. When N-fertilizer was withheld the % N in the dry matter of the different crops declined in a regular way with increasing plant weight and was always about 0.6 × the critical % N; crop weight was approximately proportional to the weight with ample fertilizer and the constant of proportionality was also about 0.6. For drilled crops, the maximum plant dry weight obtained with any level of fertilizer-N was almost linearly related to the duration of growth. These results are in approximate agreement with those predicted with a previously described simulation model. On the basis of this work, a simple field method is suggested for estimating the rate of mineralization of soil organic matter in some soils.

The apparent recovery of fertilizer-N by the different vegetable crops always declined linearly with increase in fertilizer-N over the entire range of applications which is in marked contrast to the constancy of apparent recovery by crops like cereals and grass that have more extensive root systems. In some instances, the apparent recovery of the vegetable crops also declined linearly with increase in the % N in the plant dry matter. The apparent recovery of an infinitely small amount of fertilizer-N by the different crops, when plotted against plant weight fell approximately about the same ‘diminishing returns’ type curve. The implications of these findings to the further improvement of the model are discussed.