Spring Wheat Yield Trends as Influenced by Fertilizer and Legumes

The benefits and environmental impacts of using fertilizers rather than legumes to promote more sustainable agricultural production systems are in question. The objective of this study was to demonstrate the long‐term consequences of withholding fertilizer N and P on wheat ( Triticum aestivum L.) yields, and to determine if inclusion of legume‐green manures (GM), or hay crops in rotations can maintain wheat yields. A 34‐yr crop rotation experiment being conducted at Indian Head, Saskatchewan, on a Udic Haploboroll with heavy clay texture, was used to assess the influence of fertilizer (N and P), sweet‐clover ( Melilotus officinalis L.) GM, and alfalfa‐bromegrass ( Medicago sativa L. — Bromus inermh Leyss.) hay on yield trends of hard red spring wheat grown on wheat stubble in rotations of varying cropping intensity. When no fertilizer was applied, yields of wheat grown on wheat stubble trended steadily downward for fallow‐wheat‐wheat (F‐W‐W), (decline of 0.41 bu/acre/yr) due to increased soil erosion and declining fertility; yield trends declined less sharply (0.16 bu/acre/yr) for unfertilized continuous wheat, because of less soil erosion. Inclusion of GM in an unfertilized GM‐W‐W rotation increased the yields of wheat grown on stubble, but the yield trends were still negative (decline of 0.39 bu/acre/yr), probably because legumes do not provide any P to the system. Even though the unfertilized F‐W‐W‐hay (H)‐H‐H system had low available P, it maintained stubble wheat yields at a generally constant level (decline of 0.08 bu/acre/yr) due to low soil erosion and because the haycontaining system markedly increased N‐supplying power of the soil. The addition of N and P fertilizer at rates based on soil tests resulted in a positive divergence in yield trends of wheat grown on stubble in F‐W‐W and continuous wheat systems as both available N and P were maintained at a high level. Legumes probably will require added P if they are to sustain crop production in a manner similar to that provided by responsible use of fertilizers. Research Question How important is proper fertilizer management for maintaining cereal yields in the long term? Will the inclusion of legume green manure or hay crops in rotation with cereals maintain crop production without addition of fertilizer? These are important questions for producers and society in the development of sustainable crop production systems. Objectives of this study were to determine the long‐term impact on yield trends of hard red spring wheat when N and P are applied, compared with applying no fertilizers to: (i) monoculture continuous wheat, (ii) monoculture wheat grown in a fallow system, (iii) legume green manure‐wheat, and (iv) legume grass (hay)‐wheat systems. Literature Summary Proponents of organic farming and low‐input sustainable agriculture allege inorganic fertilizers degrade soil. They recommend use of legumes to provide N, and crop rotations and tillage to reduce pests and weeds. Because legumes do not supply P, systems relying solely on legumes for maintaining soil fertility will eventually become impoverished and yields will decline. In contrast, research has shown that if inorganic fertilizers are applied according to soil tests where cereals are grown annually, soil organic matter and the N‐supplying power of soils are maintained as effectively as when legumes are included in rotations. The experiment was carried out over 34 yr (1958‐1991) on a thin Black Chernozem (Udic Haploboroll) heavy clay soil, at Indian Head, Saskatchewan, Canada. Rotation studies were fertilized (N and P) and unfertilized fallow‐wheat‐wheat and continuous wheat, and unfertilized sweetclover green manure‐wheat‐wheat and fallow‐wheat‐wheat‐hay‐hay‐hay (Table ). Hay was bromegrass‐alfalfa mixture. The legumes were seeded together with the preceding wheat crop. All phases of each rotation were present every year. Field‐size equipment was used for all operations, with tillage and herbicides used to control weeds. Applied Questions How important is proper fertilizer management for maintaining cereal yields in the long term? Yields of wheat grown on stubble (stubble‐wheat) were increased by fertilizer in both the 3‐yr and continuous wheat systems (Table ), particularly in later years when soil tests were used to determine N requirements. These yield increases reflected the concomitant increases in N‐supplying power and P availability of the soil due to addition of fertilizers (Table ). When no fertilizer was applied, wheat grown on fallow showed no trend (data not shown), but yields of stubble‐wheat declined steadily at an average rate of 0.41 bu/acre/yr (Table ). This probably was related to a depletion in soil N‐supplying power and P availability (Table ) caused by soil erosion in fallow years and net export of N and P in harvested grain. Without addition of fertilizers, there was a slight decline (0.16 bu/acre/yr) in yields of continuous wheat, but this was significantly less than that of stubble‐wheat yields in the fallow system, reflecting less erosion and higher N‐supplying power of the annually cropped system. Will the inclusion of legume green manure or hay crops in rotation with cereals maintain crop production without addition of fertilizer? Legume green manure increased stubble‐wheat yields compared to those of unfertilized fallow‐wheat‐wheat because the fixed N increased N‐supplying power of the soil. Because legumes supply no P, however, yields of the green manure system still trended downwards with time at a rate similar to unfertilized fallow‐wheat‐wheat (Table ). The hay crop, despite the removal of forage in 3 of every 6 yr, enhanced stubble‐wheat yields more than those in the unfertilized fallow‐wheat‐wheat and green manure‐wheat‐wheat systems, but yields were not as high as that of well‐fertilized monoculture wheat. The hay system performed well because it is subject to less erosion than all systems except continuous wheat, and because it markedly increased the N‐supplying power of soil (Table ). It was not as effective as well‐fertilized monoculture wheat systems, in part because the hay supplies no extra P. It is also possible that hay did not fix as much N as that applied to monoculture wheat as fertilizers, but this was not determined in this study. Recommendations In subhumid Great Plains soils, monoculture wheat, if properly managed, is a more sustainable system than including legumes in rotation with wheat without fertilizer P being added. Summerfallowing should be minimized on these soils because it degrades soil fertility causing yields of stubble‐wheat to decline over time. Fertilizer treatments over 34 yr. Average fertilizer rates Year and treatment N P ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐lb/acre‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ 1960–1977 (general recommendations) Wheat on fallow 5 10 Wheat on stubble 20 7.5 Since 1978 (soil test) Wheat on fallow 9 7 Wheat on stubble 68 7.5 23‐23‐0 and 11‐48‐0 used. 34‐0‐0‐ and 11‐48‐0 used. Average annual rate of change in yield trends, potential N‐supplying power of soil measuredin 1987, and available P measured in 1991. Potential Average yield N‐supplying Sodium bicarbonate increase, power, extractable P, Treatment bu/acre/yr lb/acre/yr lb/acre F‐W‐(W) −0.41 25 33 F‐W‐(W) (N + P) 0.45 34 50 Cont (W) −0.16 32 28 Cont (W) (N + P) 0.49 49 38 GM‐W‐(W) −0.39 39 23 F‐W‐(W)‐H‐H‐H</jats: