BUILD-UP OF SOIL FERTILITY

Press, Volume XCVIII, Issue 28820, 14 February 1959, Page 8

BUILD-UP OF SOIL FERTILITY

Trials By Grasslands Division

How long does it take to build up soil fertility with good pasture and then how long does that fertility last ?

Trials being undertaken by the Grasslands Division of the Department of Scientific and Industrial Research at Palmerston North which seek to answer these questions were discussed by the Director of the division, Dr. Peter. Sears, at the last conference of the New Zealand Grassland Association.

Dr. Sears said previous trials had indicated that at Palmerston North live years of good pasture produced soil fertility which gave top yielding forage crops. In the present series there were several pasture and fertiliser combinations all of which were being used on raw subsoil. Grass without clover or nitrogen produced only about 20001 b of dry matter to the acre even with adequate phosphate and potash, etc. White clover alone produced some 90001 b but the grass and clover combinations produced 11.000 to 14,0001 b of dry matter. Without the return of the animal manure there was clover dominance with only a slow increase of grass but with the return the pasture rapidly went to grass dominance with a greater yield and seasonal spread. An interesting and important point was that the response to mineral fertiliser was first in the form of extra clover and then later when soil nitrogen had been increased through the clover an added response to fertiliser was obtained in extra grass growth—this was particularly so with ryegrass and cocksfoot. Mineral Content Such extra and cyclic responses were quite logical when considered against the greater mineral content of grass over clover, except in calcium —but the fact was not always appreciated by farmers who worried about too much clover in early pasture stages. Instead of cutting for silage or hay and reducing fertility on such paddocks, there was an obvious strong case for continued

grazing, even with stock difficulties. arid for more fertiliser. The use of a whi ’ of spray to set the clove! back a little, as carried out by some Waikato farmers, and thus urge forward the grass, appealed as another possibility tor extreme cases.

In the Palmerston North trial there was pure grass with artificial nitrogen (urea) applied in just sufficient quantities to keep the grass up to full growth. The amount needed so far had been greater than the equivalent of one ton of sulphate of ammonia to the acre a year This gave ' another check on the large input of nitrogen by a robust white clover.

This was a useful point to bear in mind when considering seed production on pure grass—quite a difference from the handful of artificial nitrogen applied to most cocksfoot seed production areas.

The trial series had not been going long enough to give a full measure of the soil fertility increase. However, rape crops after the best pasture combinations on. this poor soil, showed yields rela- i

tive to crops on first-class soil as follows: raw soil at beginning of trial. 15 per cent., after one year 30 per cent, and after two years 50 per cent. A similar sort of situation applied in the earthworm populations and also in the general appearance and structure of the soil itself.

In the series of trials aimed at studying the exhaustion of this fertility the outstanding feature had been the rapid fall in soil fertility under the particular soil and climate at Palmerston North. The first crops on land built up by years of good pasture had yields of some 50 to 60 tons of kale or green maize, 30 to 40 tons of turnips or rape or over 25 tons of potatoes and all of these were obtained without any fertiliser. The second crops, however, showed a dramatic drop to less than half of these yields and with a continuing but slower fall in subsequent years down, to the typical stunted and yellowish crops of low fertility soil. Such declines in yield were, however, not surprising when considered against chemical composition. For example, in a 50ton crop of kale there was the equivalent of over half a ton of sulphate of ammonia, scwt of muriate of potash, and 2cwt of superphosphate. In a similarly heavy maize crop there was even more potash. Nutrient Loss Other checks within this trial series and also in other trials showed that nitrogen was the primary nutrient loss at Palmerston North, undoubtedly from the rapid breakdown of the fairly unstable organic matter. For example, on one area after taking one crop of kale, nitrogen treatments were applied to the succeeding turnip crop. With no nitrogen the turnip yield was 10 tons, with 2cwt of urea 20 tons, with 4 cwt of urea 30 tons, and with 6cwt of urea 40 tons. The 40-ton yield was the same as that obtained directly after good pasture and the 10-ton yield was the same as that after poor pasture at Palmerston North.

These trials would obviously need to run for much longer and on other major soil and climatic types, said Dr. Sears, but even allowing for’ quite a variation and also luxury consumption by crops, there was an obvious need for very careful thought about growing crops and also about crop utilisation to avoid fertility losses by stock or by carting off the paddock.