1. Figures
2. Photos
    

 The first point to acknowledge is that variability across a land does exist. Figure 1 is a yield monitor map for a land that was regarded as a uniform unit and treated as such. It is seen that in the lower southwestern corner (A) the yields are less than 4 t/ha, while just north of this (indicated as B) above 10 t/ha was recorded. In the eastern section (right hand side), areas recorded ~10 t/ha (C) and 4 t/ha to 7 t/ha (D). These differences are significant and have important monetary implications and thus it is important to know the causes. The linear strip of lower yields in the middle of the land (E) is obviously due to human error; natural influences do not have distinct transition lines, but there are transition zones.

A second principle to acknowledge was originally stated in the 1840s and is known as Liebig�s law of the minimum:

�The level of plant production can be no greater than that allowed by the most limiting of the plant
growth factors�.

This principle is very effectively illustrated by using the well-known analogy of a wooden barrel filled with a liquid (Figure 2). The level of water in the barrel represents the level of crop production, while the wooden slats are essential growth factors, e.g. light, heat and nutrients. For the first barrel, nitrogen is the limiting factor. When the nitrogen limitation is corrected (barrel on right) the level of crop production is raised until it is controlled by the next limiting factor, in this case potassium.

The essential growth factors include climate (light, heat units and moisture), physical environment (soil physical properties and situation in the landscape) and soil nutritional status (mainly nitrogen, phosphate and soil acidity). These are the factors that must be quantified when assessing the resource potential for grain production. If there is a physical limitation, this limit must be identified and accepted as the ultimate limitation since little can be done to rectify it. However, if there are negligible physical environment limitations and the limit is of a nutritional nature, this can be rectified by liming and appropriate fertilization. In this case, the question is: To what extent can one go? To treat a 3 t/ha portion the same as a 6 t/ha portion doesn�t make sense; either inputs will be wasted on the lower production portion or the full potential of the high portion will not be realised.

Considering the physical environmental factors individually, the following is known:

• Light intensity for photosynthesis is never limiting in the grain production areas of South Africa, so it does not have to be considered.
   
• There are normally ample heat units during summer for maize production in the main grain production areas. From 1 October until 31 March the growth degree days (doC) are ~ 1500 for Bethal, while in Koster the value is ~ 1700 doC, an increase of 14%. This is why planting must be done earlier (optimum planting date being mid October) in the cooler eastern regions. It is only in the Eastern Free State where heat units can be insufficient during a wet summer.
   
• Moisture is undoubtedly the most limiting factor in crop production. The summer rainfall events are high intensity
  showers with hot dry periods between events. The mid-summer �drought� is a common occurrence.

From the above it is obvious that to evaluate a soil with respect to yield potential, its ability to store and hold water so that it can supply the crop during the dry periods between rains is of cardinal importance. In previous articles published in the Senwester, the importance of soil depth and the clay content of the soil profile have been highlighted. Simply stated, soil depth determines how much water a profile can hold and clay content determines how long the water is held. If this is coupled to the relative position in the landscape (i.e. is it in a run-off position or water catchment position?), then a very good indicator of yield potential can be made. It is these aspects that the
Senwes Soil surveying team evaluates when they do a soil potential survey.

Finally, if it has been established that a soil has the capacity to store adequate water, then the nutritional status must be assessed. This can only be done through chemical analysis of soil samples that have been properly taken. One must not disregard the practice of band placing of fertilisers. Thus it is of the utmost importance that soil samples are correctly taken and a composite sample correctly representing the sampling site is sent to the laboratory.

If further information regarding soil surveying for yield potential and nutrient status is required, contact Jacques Odendaal on 083 458 1296, Johan Thiart on 082 375 8908, Koos van Rensburg on 083 440 1496 or Dr Neil.

Dr Neil du Sautoy is a senior soil scientist with Senwes Agricultural Services. Contact him at (018) 464-7391 or 082 419 0949.

Figures

Yield monitor map illustrating yield variability.

An illustration of the law of the minimum (after Brady, 1990).

Jacques Odendaal (Senwes Soil Surveying Services, left) discussing a soil core with a client.