Phil Busey Agronomy
Consulting Inc.





Vegetable irrigation practices vary between sand, muck, and rockland soils, and affect fertilization practices accordingly. Technological advances, such as drip tape and fertigation, have had a large effect on improving profitability of vegetable crops in Florida, and reducing environmental impacts of excess nutrients, particularly in sand soils vulnerable to nutrient leaching loss.

Sand soils are suitable for fresh market vegetable production in full-bed mulch culture combined with microirrigation, e.g., subsurface drip irrigation, e.g., drip tape and fertigation. Muck or humic soils are often subirrigated by seepage from lateral ditches. Rocklands are irrigated by sprinklers from shallow wells.

It is critical that soil tests be done before planting, because this may affect the recommendation for preplant fertilizers placed in the mulch bed before covering, also because once nutritional deficiency symptoms are noticed, it may be too late to save the situation. It is very important for the interpretation of soil tests that the proper extractant be used depending on soil pH.

For many Florida soils, particularly higher pH soils, the Mehlich III extraction is preferable to the commonly used Mehlich I. Mehlich III will show P levels approximately 2 x what are shown by Mehlich I. At very high soil pH, the Olsen extraction should be used for phosphorus in soil and the ammonium acetate extraction is generally preferred especially where calcium is high. In calcareous soil Mehlich III extraction overestimates total exchangeable cations. These kinds of discrepancies are a serious challenge in obtaining reasonable interpretations of soil nutrient deficiency because University fertilizer recommendations are often based on a single extraction method. For example, Mehlich I is commonly used by University of Florida publications, and this is not appropriate for all soils.

Every possible effort should be done to find a published relationship, e.g., a curve of crop yield and quality on soil nutrient availability, that was determined in careful experimental field studies of yield and quality using the same type of soil as in the grower's field. Furthermore, interpretation of the grower's soil nutrient availability should be based on soil sample extraction that is appropriate for that soil. Thirdly, that extraction, whether Mehlich I, Mehlich III, Olsen, ammonium acetate, etc., should have been the same extractant that was used in the initial experimental studies on which recommendations are based. Finally, since all three data sources, the experimental curve, soil analysis of the experimental field, and soil analysis of the growers' field, are each estimated with error, in a perfect world where the right kind of data are available, errors in estimating optimum fertilizer application rate can be substantially magnified by errors in the different data sources. And, in reality, the same method and the right method of extracting available soil nutrients is most often not done.

Thoughtful approaches are needed to deal with the lack of correspondence of soil nutrient extraction methods, and the errors inherent in the data, as well as the fact that conditions in experimental yield and quality trials may not be representative of growers' production fields. The first approach is to do on-farm research as a part of the production operation. The second approach, while much less expensive but prone to problems, is to interpret soil analysis results based on knowing the degree of overestimation of different extractions. That runs the risk of ignoring rather than solving the problem that the wrong extraction was used and there may not be a stable ratio of results from two different extraction methods across different soils. The third approach to dealing with errors in extraction and interpretation may in some cases be to use a ratio of major nutrients in addition to sufficiency studies.


Diseased pepper plants

For example, the main fertilizer nutrients of concern for production of most vegetables, depending on soil sufficiency, are N, P, K, and Mg. Soils with high Mg may need additional K; this is expressed as the Hartz ratio = exchangeable K/(sqrt(exchangeable Mg)) in meq/100g units (Hartz et al., 1999). In fruits and vegetables the ratio of magnesium to other nutrients such as potassium may be a better indicator of crop quality than the individual nutrients. See Gerendas and Fuhrs, 2013, for a more complete review of the subject of magnesium. So while ideally the right soil extractant should be used in both the experimental study and the growers field, the cations potassium and magnesium should generally extract in similar ratios across common extractants, and the Hartz ratio has utility itself and can partly get around the problems of different extraction methods.

Gerendas, J. and H. Fuhrs. 2013. The significance of magnesium for crop quality. Plant Soil 368:101-128.

Hartz, T. K., et al. 1999. Potassium requirements for maximum yield and fruit quality of processing tomato. J. Am. Soc. Hort. Sci. 124:199-204.