The paper reports the results for six factorial experiments carried out over a period of 3 years, and designed to measure the effects of nitrogen, phosphate and potash on the yield of dry matter and to determine the effect of nitrogen on the phosphate and potash requirements of grass being cut five times per annum for conservation.
Heavy dressings of nitrogen reduced the percentage of dry matter in the fresh herbage, but produced a very large increase in the yield of dry matter. There were quite large variations in the response to this plant food, but 348 lb. N per acre per annum practically doubled the yield and generally the response curves were substantially linear.
The response to nitrogen depended on an adequate supply of potash and at five of the centres there were very large interactions between these two nutrients. The need for potash was greatest at the highest nitrogen rate and was much greater in the second and third than in the first year of the experiments.
At the highest nitrogen rate some of the responses to potash were very large. The yields of dry matter showed no large or consistent differences between applying 336 lb. K2O in one dressing during the winter months or applying this amount in five equal dressings, one for each cut. The results demonstrated the ability of the soils to supply potash and showed that the regular cutting and removal of herbage, especially if heavy dressings of nitrogen were applied, exhausted the potash reserves in the soil.
Phosphate had practically no effect on the yield of dry matter either in the presence or absence of nitrogen.
With adequate potash the production of dry matter per lb. of fertilizer nitrogen was practically independent of rate and the overall mean results are between 14·0 and 15·7 lb. In the absence of fertilizer potash there was a very large effect of rate: 15·0 lb. dry matter per acre was produced per lb. N with the 87 lb. N per annum treatment, 13·7 lb. with 174 lb. N and 10·0 lb. with the 348 lb. N treatment.
Approximately 70–75% of the annual production was obtained before the end of July. Thus 1 lb. of fertilizer nitrogen produced considerably more dry matter per acre during May, June and July than later in the season.