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Neil Miles


By Nitrogen, Nitrogen Products No Comments

Afrikaans: Stikstofbemesting vir Intensiewe Weidingproduksie Nitrogen (N) is the nutrient taken up in largest quantities by pasture plants from the soil.  Its availability, together with temperature and moisture supply, are usually the major factors determining the productivity of pastures. Responses of grasses to applied N The responses of pastures to fertilizer N have been studied in scores of research trials both locally and overseas.  In South African research, the major focus has been on the N requirements of ryegrasses, kikuyu and Eragrostis curvula (weeping lovegrass), with limited work being carried out on other species such as cocksfoot, fescue and Digitaria eriantha (Smuts fingergrass).  For the relation between grass DM (dry matter) yield and fertilizer N applied, a characteristic response curve is obtained, an example of which is presented in Figure 1. When N is applied there is usually an initial near-linear response (A in Fig. 1), a phase of sharply diminishing response (B) and a point (C) beyond which N has little or no effect on yield.  The amount of DM produced for each kilogram of N applied within zone A depends largely on the species under consideration, the frequency of defoliation and growth conditions.  Tropical grasses generally produce more DM per unit of N than do temperate grasses.  In field trials, Eragrostis curvula, for example, has produced up to 60 kg DM per kg N applied, but irrigated Italian ryegrass only between 25 and 34 kg DM per kg N applied.  In the United Kingdom, perennial ryegrass produced an average of 23 kg DM/kg N over an N application range of 0 – 300 kg N/ha.  It must be emphasised that data such as these are averages over the season and conceal wide variations in response efficiency within the season.  For example, in perennial ryegrass the spring response is two to three times greater than at other times of the year. Milk production response On intensive dairy pastures, the additional feed produced in response to N fertilization is ideally converted into milk production.  A typical conversion ratio is about 15 kg pasture dry matter per kg milk-solids, or roughly one kg pasture dry matter per liter of milk. In South Africa currently, the value of pasture dry matter in dairy farm operations is estimated to be approximately R2000/ton.  In overseas studies, it has been estimated that the response in terms of milk production ranges from 9 to about 16 kg milk per kg fertilizer N applied.  This arises not because of any significant increase in yield per cow, but from an increase in stocking rate, i.e. cows per hectare. Type of fertilizer Urea and LAN (limestone ammonium nitrate) are the two most important forms of fertilizer N used on pastures, with other products such as ammonium sulphate being used in lesser amounts.  Grasses take up N in both the ammonium and nitrate forms; however, since ammonium (including the N in urea) is converted to nitrate within a few weeks in well-aerated non-acidic soils at temperatures above about 5˚C, most of…

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By Soil Health No Comments

Soil compaction can be a serious yield-limiting factor in crop production, and indications are that this problem is more widespread than is generally perceived. Since compaction is below ground and, thus, not visible, its role in yield decline often goes unnoticed. Soil types Soils vary in their susceptibility to compaction. Research indicates that soils with higher sand and silt contents are more vulnerable to being compacted than loam and clay soils. In addition, the higher levels of organic matter in humic soils reduce their susceptibility to compaction. Causes Management practices that promote compaction include the following: Intensive tillage. Tillage may temporarily loosen the soil; however, in the long-term regular tillage increases the bulk density of soils through the weakening of soil structure and the depletion of soil organic matter. In addition, implements such as the mouldboard plough and disk harrow compact the soil beneath their working depth. Repeated use of these implements can result in the development of plough pans (compacted zones immediately below the ploughed layer (Figure 1, left). Wheel traffic. The wheels of vehicles used to apply fertilizers, lime and other products, harvest crops and pull implements, result in decreases in soil porosity and yield-limiting compaction (Figure 1, right). In the production of annual crops, 60% or more of the soil surface can be trafficked through cultivating, planting, fertilization, herbicide and other chemical applications (Mitchell & Berry, 2001). The in-field traffic associated with maize silage operations and the production of sugarcane frequently results in severe compaction problems (Figures 2 and 3) and associated yield losses. Animal hooves. The role of animals, particularly in intensive grazing systems, in promoting soil compaction is often overlooked. However, although animals are not as heavy as machinery, their weight is applied to the soil in a relatively small hoof-print, and so can cause significant compaction and damage to pastures, thereby decreasing forage yields and animal performance (Figure 4). Importantly, soils are most susceptible to being compacted when they are wet. Unfortunately, timing of farming operations, such as harvesting, fertilizing and rotating livestock, often makes it difficult to exclude machines and animals from soils when wet. Effects of compaction Detrimental impacts of compaction on soil health and plant growth are due largely to the decrease in the proportions of macro-pores in compacted soils. Effects associated with this include the following: Root penetration into compacted soil layers is restricted, and root and biological soil health compromised through water logging and anaerobic conditions in soils. Water infiltration as well as water-holding capacity of soils are reduced, and effective rainfall is diminished through increased runoff. Ponding on the soil surface after rain or irrigation is usually an indication of compaction. Nutrient uptake by roots is poor in compacted soils. This is due to, firstly, limited root development, and secondly, restrictions on the movement of nutrients to roots by mass flow. Yellowing of plants due to nitrogen deficiency is often associated with poor nitrogen recovery by roots in compacted soils. The picture in Figure 2 (left) provides clear…

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