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Field studies in California have indicated that soils and plants with high levels of potassium presented a lower incidence of YSD. Although soil nutrient levels cannot prevent YSD, they can indicate a higher or lower probability of occurrence of this disorder when location is concerned. It is important to know your soils in order to increase the risks of YSD. The most desirable characteristics of the soil for low risk of YSD would be:

 Midwest and Mid-Atlantic Soils

California Soils

Extractable K is greater than 0.4 cmol/Kg

Extractable K is greater than 0.7 cmol/Kg

The Hartz ratio is greater than 0.35

The Hartz ratio is greater than 0.25

K is at least 4 % of the CEC

K is at least 2% of the CEC

Ca/Mg ratio (meq/100g) is at least 3:1

Ca/Mg ratio (meq/100g) is at least 2:1

 

To check the status of your soil and estimate the "Hartz" ratio click here.

 

Frequently Asked Questions.

 Can Mg levels in soil be too low?

Yes. According to the Ohio Agronomy guide bulletin 472 the minimum exchangeable soil Mg is 50 ppm. Low levels of Mg can occur mainly due to low total and exchangeable Mg in coarse soils of humid regions. In these soils, loss of Mg is aggravated by low fixation of Mg to soil particles and leaching by water.

Even though soils may present a considerable level of available Mg, deficiency can occur due to lower uptake of Mg by plants. High levels of exchangeable potassium, calcium or aluminum in soils reduce uptake of Mg by plants. Adding too much K or Ca to soils with less than 125 ppm of Mg can lead to deficiencies. Also, a leaf test of less than 0.2% Mg dry weight may indicate that your plants have a Mg deficiency.

 

Should I apply twice as much Potassium fertilizer to decrease risk of YSD?

No. Every application of fertilizer should be based on the recommendations of your soil test.  Excessive application of K fertilizer is economically undesirable, since not all K applied in the soil will be readily available to plants. Several factors cause K loss in soils:

  1. K can be fixed by soil colloids. Fixation is a factor for fine textured soils when the concentration of K is high. Fixation may prevent potassium from leaching by water, but at the same time, reduces the amount of K available to the plants.
  2. Leaching of available K by water. Leaching is a factor for coarse soils with low CEC and humid environments. In these cases, split applications of fertilizer decrease loss of K by leaching.

 

Can Potassium levels in soils be too high?

Yes. Excess potassium is due to high input of fertilizers or manure in the soil. Tomato growers should be especially aware if rotation crop is intended for forage. High concentrations of potassium can reduce magnesium uptake by plants, affecting plant physiology and the metabolism of animals that consume mainly forages. The recommended Mg:K ratio is 2:1 on a percent exchangeable basis. To know more about forage fertilization, access the following links: Ohio Agronomy Guide Bulletin 472: Forage Production (http://ohioline.osu.edu/b472/forage.html) and Forage Management Tips (http://ohioline.osu.edu/agf-fact/0016.html)

 

What are the desirable soil test results for tomatoes?

There are few guidelines for desirable nutrients values for tomatoes, and those are  not always applicable to your soil. You can find some information on the sites below. These are just guidelines and should not be put into practice without a complete soil test and recommendations from a specialist.

 

For more information on soil testing click below:

 

 

Potassium: absorbed by plants in the ionic form K+. It is important in the formation of carbohydrates and proteins; also it helps regulate the water within the plant cell, among other functions. Supplied to plants by soil minerals, organic materials and fertilizers.   Calcium: absorbed by plants as divalent cation Ca++. It is important in root and leaf development, acting on the transport and retention of other elements. Play important role on plant strength. Supplied to plants by soil minerals, organic materials and fertilizers and liming materials.   Magnesium: absorbed by plants as divalent cation Mg++. It is an essential constituent of chlorophyll, and activates photosynthesis and plant enzymes. Supplied to plants by soil minerals, organic materials, fertilizers and dolomitic limestone.
Cation Exchange Capacity: sum of exchangeable cations (ions with positive charge like K+,Ca++, Mg++) that a soil or material can hold. Soils generally have a negative charge, holding and preventing excessive leaching of cations. Absorbed ions can be released with time to the soil solution and can be absorbed by the plants.  Usually, sandy soils have the lowest CEC, and clay loams, clay and organic soils have the highest.

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