Experiments on a sandy loam, using adequate fertilizer, gave:

(a) Crop yield (as total dry matter) when soil water content was kept near field capacity.

(b) A measure of the departure from field capacity (limiting deficit, Dι) that produced no detectable change in yield.

(c) Responses to irrigation: (i) conventional, as δY/δI; (ii) a theoretical maximum, k, after allowing for Dι in the water balance.

(d) Other information, botanical and technical.

Grass (S 22 Italian ryegrass)

Two year crop, cut fourteen times, October 1959 to October 1961.

(a) Best yields: 1960, 15 t ha−1 (ɛ ≃ 100 × 10−4); 1961, 12 t ha−1 (ɛ ≃ 70 × 10−4).

(b) Dι ≃ 5 cm.

(c) (i) δY/δI: 1960, 0·36; 1961, 0·43 t ha−1 cm−1; (iii) k: 1960, 0·40; 1961, 0·27 t ha−1 cm−1.

(d) In 1960 irrigation was needed in spring only: the benefit was maintained on plots with large N and K dressings, but almost disappeared on those with small dressings (Y = 11 t ha−1). In 1961 the unirrigated crop could not exploit all the limited summer rain: hence δY/δI > k.

One year crop, cut five times, March to October 1965.

(a) Yield, 12 t ha−1 (ɛ ≃ 90 × 10−4, from emergence).

(b) and (c) (i) The meteorological need was slight, Dι (above) was not exceeded, and there was no response; (c) (ii) k = 0·46 t ha−1 cm−1.

Lucerne (Du Puits)

Three year crop, cut eleven times, April 1962 to November 1964.

(a) Best yields: 1962, 7; 1963, 8; 1964, 10 t ha−1.

(b) Dι = 11 cm for established crop.

(c) (i) No response other than in first dry spring of establishment. At first cut δY/δI = 0·18 t ha−1 cm−1, (ii) k = 0·26 t ha−1 cm−1.

(d) Extra K seemed to be helpful in sustaining the early benefit throughout the 3 years.

Clover (Crimson)

One year crop, drilled April and cut (once only) July 1963.

(a) Best yield: 3 t ha−1.

(b) Dι, not detectably different from zero (evidence inadequate).

(c) (i) δY/δI = 0·16 t ha−1 cm−1; (ii) k ≃ δY/δI (?).

(d) First cut completely defoliated irrigated crop, there was no recovery, and the experiment was abandoned.

Clover (Dorset Marl)

One year crops undersown in previous barley crops, 1963 for 1964 (three cuts), 1964 for 1965 (three cuts).

(a) Best yields: 1964, 9 t ha−1 (ɛ ≃ 60 × 10−4); 1965, 8 t ha−1 (ɛ ≃ 60 × 10−4).

(b) Dι ≃ 2·5 cm.

(c) (i) δY/δI: 1964, 0·24 t ha−1 cm−1. 1965, no response; (ii) k: 0·22–0·34, depending on method of derivation.

(d) The response to irrigation was slightly greater on plots not previously irrigated in the barley year. In (c) the uncertainties arise out of excessive rain after the first irrigation in 1965; there was estimated drainage from the irrigated plots but not from the control plots. In 1964, the control plots failed to exploit all the summer rain.

General

There seems to be a need to know two limiting deficits, one, the Dt of these papers, that is a measure of what can be tolerated without any check to growth, and a second that is a kind of maintenance deficit, necessary to keep the crop non-senescent so that it can use all the rain it gets.

Irrigation greatly helped the establishment of all three leys, and both grass and clover responded well to later irrigation. Lucerne did not. In English units, the limiting deficits, Dt, for well-fertilized established crops were, approximately, 4 in for lucerne, 2 in for grass, and 1 in for clover.