The effect of the level of water deficit on the yield of grain and total above-ground biomass of maize planted at greatly differing densities was investigated in a strictly controlled field experiment, repeated for four seasons. Differences in water supply were obtained by the seasonal variation in rainfall and by supplementing the rainfall with irrigation in some of the treatments. In each season the highest level of water represented conditions of adequate soil water supply. An abundant supply of inorganic nutrients enabled the study to be confined to an investigation of the interaction between level of soil water supply and planting density on the yields of maize. At adequate levels of soil water supply yields of grain and total above-ground biomass increased over the whole range of planting densities. The highest density employed (111 111 plants ha-') gave rise to a LAI in excess of 8. Reduction in the supply of soil water caused a reduction in the critical planting density for maximum biomass production, but did not affect the asymptotic nature of the relationship between biomass yield and planting density. The relationship between the grain yield and planting density was influenced by a deficient level of soil water in two ways: firstly, by changing the nature of the yield response from nearly asymptotic to distinctly parabolic, and secondly, by reducing optimum density to lower levels as water stress became more severe, Die invloed van peil van water tekort op die opbrengs van graan en totale bo-aardse biomassa van mielies is ondersoek oor 'n wye reeks plantdigthede in 'n streng gekontroleerde veldproef wat vir vier seisoene herhaal is, Verskille in beskikbare grondwatervoorraad is bepaal deur aanvullende besproeiing en seisoenale variasie in reenval. In elke seisoen het die hoogste peil van beskikbare grondwater toestande van vo/kome genoegsaamheid van water verteenwoordig, 'n Oorvloedige voorraad van minerale voedingsstoffe het gesorg dat die ondersoek beperk kon word tot 'n studie van die wisselwerking tussen peil van water en plantdigtheid in terme van die opbrengs van graan en bo-aardse biomassa, Waar 'n genoegsame peil van beskikbare grondwater gehandhaaf is, het die opbrengste van graan en bo-aardse biomassa oor die hele reeks van plantebestande toegeneem, Die digste aanplanting (111 111 plante ha-1) het 'n BOI van meer as 8 tot gevolg gehad, 'n Vermindering in die beskikbaarheid van grondwater het 'n vermindering in die kritiese digtheid vir biomassa veroorsaak sonder dat dit die asymptotiese vorm van die verband tussen opbrengs van biomassa en plantdigtheid be'invloed het. Die verband tussen graanopbrengs en plantdigtheid is deur watertekort op twee wyse be'invloed, ten eerste is verband tussen opbrengs en plantdigtheid van 'n asymptotiese tot 'n duidelike paraboliese verband verander, en ten tweede is die optimum plantdigtheid verminder na mate die watertekort vererger het.

Reports on the effect of soil moisture stress on plant uptake of P have been conflicting. The effect of the dynamic variation in soil moisture on P uptake by soybeans (Glyfine max L. Merr.) was studied in vivo. A split‐root technique with plants grown in 3 liter containers in the greenhouse was used whereby moisture stress was localized in the zone having the only source of P for the roots, while water and P‐free nutrients were supplied through the other portion of the root system. The procedure minimized the effect of plant‐water deficit. Differences between moisture treatments accounted for 81.5% of plant P content when the effects of fertilizer treatments were removed, while variation in soil moisture content within each moisture treatment accounted for only 8%. The extent P uptake was impaired by high moisture stress was conditioned by the P status of the plant which in turn was determined by the previous soil moisture history of the zone supplying the P. After watering, plants exposed to a high moisture stress resumed P uptake at a rate exceeding that of plants absorbing P from the adequately watered compartment. The range of response to which increasing levels of fertilizer P was obtained increased as the period of moisture stress in the fertilized zone was prolonged. Temporary luxury consumption of P, induced by high levels of soil P and foliar application, benefited soybeans whose P supply was curtailed by high moisture stress. Moisture stress appeared to affect P uptake largely through its influence on diffusion rather than mass‐flow. It is suggested that the inconsistencies of field results reported in the literature studying the relationship of P uptake and moisture stress could be caused by the peculiar P uptake behavior found in these experiments.

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