Production systems involving interplanted food crops are widespread in tropical latitudes, and interest in quantifying the productive potential of intercrops is high. Published data often suggest a sizable gain in land‐use efficiency by growing such crops in mixtures. Our thesis is that many of the large intercrop advantages reported in the literature are misleading because the conceptual basis on which the monoculture‐vs.‐intercrop comparisons were made is incomplete. In this paper, we review the various methods for estimating intercrop productivity, propose a concept which we believe will remove the principal limitation in conventional methodology, and test the new concept with intercrop data from the literature. The land equivalency ratio (LER) is the most‐used convention for intercrop‐vs.‐monoculture comparisons, but LER is frequently inappropriate because cropping‐system duration, i.e., time, is not included in its calculation. Duration of land occupancy by an intercrop is often longer than production‐cycle duration for one or more of the interplanted species. We propose an area‐×‐time equivalency ratio (ATER) and suggest that it will correct this conceptual inadequacy in LER. When published intercrop data were reevaluated via the ATER concept, large land‐use advantages ascribed to growing food crops in mixtures disappeared. We conclude that most crop mixtures utilize land area and time (area·time) at about the same efficiency as pure stands of the mixture's components.
Phosphorus reserves in Ultisols are inherently low; but many Ultisols along the Atlantic seaboard are now high in P, both extractable and total, because P additions have exceeded P removal for many years. How long a high‐P soil will maintain plant‐available P above yield‐limiting levels is of agroeconomic relevance. A field experiment initiated 35 yr ago on Portsmouth soil (fine sandy over sandy or sandy‐skeletal, mixed, thermic Typic Umbraquult) and monitored for crop yields and soil‐test P (Mehlich‐1 extractant) during 8 yr of active P buildup and 26 yr of residual decline has provided quantitative data on this issue. Yields of corn (Zea mays L.) or soybean [Glycine max (L.) Merr.] were maximal with soil‐test P ≥ 22 g m−3, and extractable P was maintained in this range (20‐24 g m−3) when P removed in harvested products (16 kg ha −1 yr−1) was replaced annually as band‐applied fertilizer. High soil‐test levels (≥50 g m−3) could not be maintained by annual replacement of crop‐removed P because P reversion to unextractable forms was a larger factor than crop removal in depleting the extractable‐P pool. Regardless of initial level, P disappearance into these unextractable forms was best described via equations having the form of a first‐order chemical reaction; but the magnitude of the rate constant varied with size of the extractable‐P pool, i.e., high‐P soils have large rate constants; low‐P soils have small rate constants. A Portsmouth soil testing 50 to 60 g P m−3 today will test above 22 g m−3, the approximate critical level for corn, for the next 8 to 10 yr without further P additions. Doubling the initial soil test will not double the time to reach yield‐limiting P levels, however; the same soil with 100 to 120 g P m−3 initially will drop to 22 g m−3 in about 14 yr.
Intercropping may be one way to increase the productivity of land. If so, it is of scientific as well as practical interest to know how the increase is achieved and devise methodologies which will exploit such systems to the fullest. This report compares the productivity of two component intercrops with that of their respective monocultures under differing nitrogen regimes. Two soil conditions were tested in each of 2 years—a poorly drained Portsmouth fsl (Typic Umbraquult, fine‐loamy, mixed, thermic) and a well drained Orangeburg sl (Typic Paleudult, fine‐loamy, silicious, thermic). Treatment variables were cropping system (intercrops vs. monocultures), row arrangement, and N rates from zero to 270 kg/ha. Corn (Zea mays L.) was used as the overstory crop and soybeans (Glycine max L. Merr.), snapbeans (Phaseolus vulgaris L.), or sweet potatoes (Ipomea batatas Lam) as the interplanted understory species. All interplanted species were at the same population as their monoculture checks. With adequate N, monoculture corn in conventional rows (97 un) averaged 9,400 kg/ha. Planting monoculture corn in paired 46‐cm rows, with 147 cm between row pairs, caused a 12% reduction in yield. An additional reduction of 5 to 10% resulted when soybeans or snapbeans were interplanted into corn in the 147‐cm space between row pairs. Monoculture soybeans yielded 2,750 kg/ha with only minor variations among sites. Interplanted soybean yields were site and year dependent, however, and varied from 33 to 55% of their respective monocultures. Intercrop soybean yields were inversely related to the amount of N applied to corn. Yields of spring‐planted monoculture snapbeans ranged from 7,000 to 15,500 kg of fresh pods/ha. Regardless of yield level, however, interplanted snapbeans yielded 48% of the monoculture check. Fall snapbeans, relay interplanted into maturing corn in mid‐August, also produced about 50% of a crop. Land equivalent ratios (LER) of 1.20 to 1.40 thus suggest a 20 to 40% increase in total productivity during one 200‐day season by intercropping. Higher production by intercrops was most easily interpreted in terms of total leaf exposure. Monoculture corn had a leaf‐area duration (LAD) of 150 days; but when soybeans were interplanted into corn, LAD of the intercrop exceeded 300 days.
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