IPI International Potash Institute
IPI International Potash Institute

Research Findings: e-ifc No. 40, March 2015

Photo by A.C.C. Bernardi

Soil Fertility Management and Weed Occurrence in Alfalfa Pasture

Bernardi, A.C.C.(1)
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(1)Embrapa Pecuária Sudeste, C.P.339, CEP 13560-970, São Carlos, SP, Brazil, alberto.bernardi@embrapa.br

Introduction

Grazed pastures in Brazil provide the major source of food for beef and dairy cattle. For this reason, well established, properly managed pastures of high productivity are essential in order to support adequate gains in animal weight. Soil fertility is one of the most important controllable factors for determining forage yield and quality. Alfalfa (Medicago sativa L. ), which is highly important for forage production in Brazil, is extremely demanding in soil nutrients, so that the provision of an adequate supply of nutrients is essential to maintain high forage quality and profitable yields (Moreira et al., 2008; Bernardi et al., 2013a,b). Interestingly, this crop requires higher levels of soil fertility than other tropical pastures (Bernardi et al., 2012).

The most common nutrient inputs for alfalfa are lime, phosphorus (P) and potash (K) fertilizers in the high weathered, low-fertile and acids soils of tropical regions (Moreira et al., 2008; Bernardi et al., 2013a,b). No nitrogen (N) fertilizer sources are used in the alfalfa production system in Brazil since all N is supplied by biological fixation by Sinorhizobium meliloti. Liming is essential for growing alfalfa in order to increase soil pH because of the sensitivity of the crop to soil acidity; the recommended pH range being between 6.5 and 7.5 (Honda and Honda, 1990). In addition, liming has other beneficial effects including raising the base saturation of the soil to 80% (Moreira et al., 2008), reducing the aluminum (Al) and manganese (Mn) toxicity as a consequence of increased soil pH, and thereby also increasing the availability of the macronutrients calcium (Ca), magnesium (Mg), K and P. Liming also favors organic matter mineralization, increasing the efficiency of symbiotic N fixation, fertilizer use efficiency and microbiological activity in the soil. Phosphate and micronutrient fertilizers are usually applied once a year. In tropical soils, P can be immobilized as precipitated phosphates of iron (Fe) or Al, or adsorbed phosphate on soil particles. This adsorption is reversible and phosphate is released into the soil solution with increasing soil pH from acid conditions by liming (Bernardi et al., 2012; Berg et al., 2005; Sarmento et al., 2001). Three forms of K (unavailable, fixed and exchangeable) are present in soils. Potassium considered readily available to plants is that in soil solution, which is in rapid equilibrium with K held on exchangeable cation sites of the soil complex. Potassium fertilizer, as muriate of potash (KCl), is usually applied after each grazing or cut, thereby avoiding loss of K to the crop as a result of K leaching through the soil profile, which can be a problem on these tropical soils of low cation exchange capacity (CEC) (Bernardi et al., 2012, 2013a., 2013b).

In addition to soil fertility, the main limiting factors for cultivation of alfalfa in tropical regions are weed management, cultural practices and grazing management (Honda and Honda, 1990, Vilela et al., 2008). Adequate soil fertility in established alfalfa pasture allows the forage to compete more effectively with weeds (Peters and Linscott, 1988; Lillak et al., 2005). Weed interference may be responsible for 30-40% reduction in agricultural production in the tropics (Vilela, 1992). Weeds may lead to a decrease in alfalfa biomass production due to allelopathic effects and competition for water, light, nutrients and space (Peters and Linscott, 1988; Moyer, 1992). In addition to competing with alfalfa forage, weeds may also serve as hosts for pests and disease, and, moreover, may complicate grazing or mechanical harvesting causing losses of both pasture productivity and quality. Little is known in Brazil concerning alfalfa used in the dairy cattle system and the interaction between soil fertility and weed occurrence. Both crop and weed species are affected by changing levels of soil fertility. According to Liebman and Davis (2000), the differences among species involving root and shoot responses to nutrient enrichment, supply of fertilizer, or lack of supply, could shift the balance in competitive relationships between crops and weeds. During the establishment of the alfalfa crop, if soil fertility is adequate and the crop is managed well, high vigor seedling growth should occur. However, the problem of weeds may worsen during the maintenance phase of the alfalfa pasture. Previous investigations have shown that weeds may invade established alfalfa stands if they become thin (Peters and Linscott, 1988).

In the present paper, the effect of soil amendment with lime and gypsum and K fertilizer on a typical low fertile, highly weathered, acid tropical soil is evaluated in relation to alfalfa pasture yield and the occurrence of weeds.

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