Experiments on a sandy loam, using adequate fertilizer gave:

(a) Crop yield when soil water content was kept near field capacity.

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

(c) Responses to irrigation: (i) conventional, as δγ/δΙ (ii) a theoretical maximum, k, after allowing for De in the water balance.

(d) Other information, botanical and technical.

Sugar beet

(a) Best yields of sugar: 1963, 9; 1963, 10; 1964, 8 t ha-1.

(b) Limiting deficit can increase to ca. 10 cm by end of September.

(c) (i) Best δγ/δΙ. 1963, 0·18 t ha-1 cm-1 (some need); 1964, 0·62 t ha-1 cm-1 (great need); 1965, slightly negative (no need); (ii) inferred k = 0·4 t ha-1 cm-1 for sugar, and ca. 1·0 t ha-1 cm-1 for total dry matter. For the period sowing to harvest the values of e are 0·7 and 1·8 x 10-2 respectively.

(d) Excess water (as in 1964) seems to depress the sugar yield slightly.

Potatoes, main crop (part of an experiment on control of nematodes, under continuous cropping)

(a) Best fresh weight yields of tubers: 1966, 46 (without irrigation); 1967, 38 (with irrigation); 1968, 33 (with irrigation), t ha-1.

(b) Evidence inadequate.

(c) Responses were small, or negative; (i) best response, in 1967, 0·7 t ha-1 cm-1; (ii) evidence inadequate.

(d) Decreased yield in 1966 may be caused by early summer leaching: the effect was about equal to removing half of the nitrogen fertilizer applied.

Potatoes, early (Arran Pilot)

(a) Best fresh weight yields of tubers: 1960, 32; 1961, 37; 1962, 16 t ha-1, all at larger of two nitrogen dressings, and after normal cultivation.

(b) De ≃ 2·5 cm.

(c) (i) Best δγ/δΙ. 1960, 1·7; 1961, 2·0; 1962, 1·1 t ha-1 cm-1, (ii) k = 1·8 t ha-1 cm-1 for nitrogen at 0·6 cwt acre-1; k = 2·11 ha-1 cm-1 for nitrogen at 1·2 cwt acre-1. Inferred values of ɛ are 1·15 and 1·45 x 10-2.

(d) Weed control by chemical means was not successful: all yields and responses were decreased by about one-third.

Barley, spring (Proctor, Maris Badger)

After early potatoes, some plots has a crop of trefoil, ploughed in (crops of 1961–3). In 1968, barley was test crop in an experiment on fumigants. Cropping in 1960 and 1964 was normal.

(a) Best yields of grain, 3·4–4·8 t ha-1 (without trefoil); 4·1–5·4 t ha-1 (after trefoil).

(b) De ≃ 4 cm before ear emergence, and unimportant thereafter.

(c) (i) Best δγ/δΙ, in 1962, ca. 0·20 (no trefoil), and 0·30 (after trefoil) t ha-1 cm-1, both at upper of two nitrogen dressings; (ii) k (for variety Proctor only, and no trefoil) ≃ 0·20 t ha-1 cm-1, and may be independent of nitrogen treatment.

(d) Previous management of early potatoes did not affect barley yields. For 1961–3—average yield of all treatments = 3·53 t ha-1—the average responses were: to trefoil, 0·46 t ha-1; to nitrogen, 0·76 t ha-1; to water 0·95 t ha-1. Trefoil halved the response to nitrogen, and may have increased the response to water.

Within limits, water and nitrogen seem to be interchangeable as management factors.

Wheat, spring (1965, Opal, in a normal experiment. 1966, 1967, Kloka in an experiment with a dwarfing compound, CCC. Four levels of N fertilizer)

(a) Best yields of grain, 1963, 4·5; 1966, 5·8; 1967, 5·9 t ha-1.

(b) De ≃ 4 cm before ear emergence, and ca. ET/4: thereafter.

The evidence is flimsy.

(c) (i) Best δγ/δΙ. 1965, 0·11; 1956, 0·16; 1967, 0·12 t ha-1 cm-1; (ii) evidence is inadequate to derive values of k, but it seems to be very small with N1 (0·4 cwt acre-1 N), and perhaps of order 0·2 t ha-1 cm-1 at all the bigger dressings (2–4 x N1).

(d) On average, CCC had no effect on grain yields, but it decreased straw yields. In 1967 many sub-plots gave very large yields: the average of nine was 12 t ha-1 as total dry matter; three were not irrigated, and had no CCC; three had CCC and were irrigated. None was at the smallest N treatment. Spring wheat responds to water like any other crop when given enough N (ca. 0·8 cwt acre-1 N, or a little more). The need continues after ear emergence.

Beans, winter 1960, spring 1961, 1962, 1968

(a) Best yields of grain, 1960, 3·7; 1961, 3·1; 1962, 3·6; 1968, 3·0 t ha-1.

(b) De ≃ 4 cm.

(c) (i) Best δγ/δΙ. 1960. 0·18 (late watering); 1961, 0·15 (early watering); 1962, 0·22 (late watering); 1968, 0·02 (early watering and then rain in excess); (ii) k = 0·17 t ha-1 cm-1.

(d) There is no evidence to show that irrigation is more important either before or after flowering (early and late, in (c)).

General

Exception for the main-crop potatoes, all crops responded to irrigation as expected, with gains of 50–100% in the driest summers. In English units, the values of the limiting deficits were, approximately, up to 4 in by the end of September for sugar beet, 1 in for early potatoes, 1½ in for barley before ear emergence, 1½ in for wheat before ear emergence and up to about 3 in 5 weeks later, and 1½ in for beans.