Chemical fertiliser mixtures are described by a formula printed on the bag, for example, 3:2:1 (25). This is the ratio of nitrogen to phosphorus to potassium. The concentration of the combination of the three nutrients expressed as a percentage of the bag is described by the (25). In other words, in every 100kg of 3:2:1 (25) fertiliser, there is 25kg (25% of 100kg = 25kg) of nitrogen, phosphorus and potassium.
The higher the number in brackets, the more concentrated the mixture. So, a 50kg bag of 3:2:1 (30) fertiliser will contain more nutrients than a 50kg bag of 3:2:1 (25) and is thus more expensive. Kraal manure can be described by the same formula. In the central areas of the Eastern Cape, for example, the average formula for kraal manure is 3:1:4 (3). In every 100kg of manure, there is a total of 3kg of nutrients in the ratio three parts nitrogen, one part phosphorus and four parts potassium.
The nutrients in kraal manure are thus one-tenth of the concentration of the chemical fertiliser mixture 3:2:1 (30). So, you would have to apply 1 000kg of kraal manure to provide the same amount of nutrients found in 100kg of chemical fertiliser.
The concentration in kraal manure depends on several factors. The most significant of these is the manure’s soil content. The more soil in the manure, the lower its nutrient concentration, and the more you have to apply.
Staying with our Eastern Cape example, kraal manure was found to contain between 20% and 80% soil by mass, causing the formula to range between 3:1:4 (7), where the soil content was low, and 3:1:2 (2), where more soil was present.
For practical purposes, the recommendations provided are based on kraal manure containing about 60% soil by mass with a formula of 3:1:4 (3). This should apply to most of the country.
Should your manure contain less soil, you can reduce the amount of manure recommended, but there will be no harm in following the suggested application rates.
Signs of nutrient shortage
If you have been harvesting crops and not fertilising, your soil will be depleted of nutrients. This will be obvious from your crops: they will grow slowly, have a stunted appearance (small plants with thin stems and small leaves), be pale green or yellowish, and deliver a poor yield. If this is the case, you should apply manure by broadcasting it – in other words, spreading it evenly over the surface. To restore your soil to a healthy, productive condition, apply 250 wheelbarrow-loads of manure/ha or one wheelbarrow-load for every 40m² of land.
An area of 40m² is more or less equivalent to a square area six large paces wide and six large paces long. A hectare is roughly the size of a soccer field, covering an area of 10 000m². To determine the number of wheelbarrow-loads of kraal manure needed, divide the area of your land (m²) by 40. For example, if you have a plot 70m long and 70m wide, its area will be 70m x 70m = 4 900m². Divide this by 40 (as you’ll need one wheelbarrow-load for every 40m² of land), and you’ll see that 122,5 wheelbarrow-loads are required.
Fertilising your crop
Crops differ in the amount of nutrients they remove from the soil. The higher the yield of the crop, the more nutrients it removes from the soil. Table 1 and 2 shows the recommended application rates for manure for certain frequently planted crops. Table 1 is useful if you intend to produce a crop on a land; Table 2 is for home gardens. For each crop, or group of crops, a low and a high target yield is given. If you are farming in an area of low, unreliable rainfall, use the application rate of manure for the low target yield. If you are in an area of high rainfall or are using irrigation, select the application rate of manure for high target yields.
CAUTION: Do not use manure on carrots. It causes them to split and reduces their quality.
Source: Using Kraal Manure as Fertiliser, W van Averbeke and S Yoganathan, Agricultural and Rural Development Research Institute, Fort Hare.