Potassium Fertility of Salinas Valley Soils

Potassium is a critical nutrient in vegetable production in the Salinas Valley. In nearly all key vegetable crops that are produced here, the amount of potassium removed in the harvested crop is similar to or exceeds that of nitrogen (Table 1). Potassium dynamics in the soil are distinctly different from nitrogen, and the need for it can be assessed by a soil test. Ammonium acetate extraction is the most common technique for assessing potassium availability in the soil. Some laboratories use the Mehlich-3 test, which gives numerically a similar result to the ammonium acetate method. In general crops growing in soils with test values above 200 ppm do not respond to potassium fertilization (Table 2).  In general, the decomposed granite soils on the eastside tend to have the lowest potassium levels in the Salinas Valley, while clay loams and clays tend to have higher values (Table 3). However, due to fertilization practices there are some exceptions noted in the 2010 survey of Central Coast soils.

However, there are some details about potassium that need to be kept in mind; the standard soil tests for K (ammonium acetate and Mehlich-3 analytical methods) provide  an index of available potassium and not a quantitative measure of soil potassium content. Other cations in the soil solution (calcium, magnesium, sodium, ammonium) can compete for plant uptake, and therefore it can be helpful to look at the percent potassium of exchangeable soil cations. Cation competition with potassium is not a problem if potassium makes up more than 3% of exchangeable cations. In soils where potassium makes up less than 2% of exchangeable cations, potassium uptake may be restricted. Table 3 shows examples of soils with relatively high potassium levels; in some of these soils potassium is relatively low as a percent of exchangeable cations and may be of concern.

Potassium uptake by plants is affected by the density of the root system of the plant. Factors that affect root development affect potassium uptake: rooting depth, irrigation system (drip vs sprinkler), soil compaction and root disease. In general, any production practice or unfavorable soil condition that reduces rooting density will reduce potassium uptake. An advantage of drip irrigation over sprinkler is the ability to deliver a concentrated dose of potassium by fertigation at the time of the season when demand is highest.

Certain soils have a tendency to fix potassium and make it unavailable or only slowly available to plants.  Soils that contain large amounts of the silicate mineral vermiculite have the ability to trap the potassium ions between layers of the crystalline structure of the mineral, thereby making it unavailable to plants. Vermiculite is generally associated with soils derived from granite, but the type of granite affects the levels of vermiculite. In addition, the age of the alluvium and the degree of weathering impacts the quantity of vermiculite in the soil. Very young soils may not have had sufficient time for all the parent material potassium (in mica and hydrous mica) to weather out of it, and will not have a significant K fixation potential. On the other hand, in very old soils, the vermiculite may have weathered to a different type of clay (smectite) which does not have the proper crystalline structure to fix potassium. Soils with high levels of potassium (e.g., > 200 ppm) are likely low in vermiculite and have little or no potential to fix potassium.

Potassium fixation potential has been identified in some soils used for cotton and other crops on the east side of the San Joaquin Valley but has not been examined in other parts of California. In a recent survey of a small number of soil samples from Salinas Valley vegetable field, none were found to fix potassium in a UC Davis laboratory test for this. The lack of fixation capacity in Salinas Valley soils may reflect the impact of crop production and past fertilization practices. Crop production may affect the weathering of vermiculite on the surface layer of soil. Fertilization practices over the years result in higher soil potassium levels which can saturate potassium fixing sites in the soil minerals.

Potassium is also capable of leaching from soils. We conducted an evaluation of leaching during winter storms on a sandy loam soil on the eastside of the Salinas Valley. The amount of anions and cations leached was measured using suction lysimeters placed two feet deep in the soil; in addition, we measured the quantity of moisture moving through the soil during each rain event. The data indicates that there was movement of potassium and other ions deeper in the soil profile (Table 4). It is interesting to note that, the relative loss of potassium was much less than other cations. Potassium is bound to the negatively charged sites on clay and soil organic matter and losses of potassium due to leaching are generally assumed to be low rich in these materials.

In general, there appears to be robust levels of potassium in some Salinas Valley soils. This is particularly true for heavier soils. However, crop removal by many Salinas Valley crops is also quite robust. Fertilization with potassium does not have the environmental consequences that we observe with nitrogen and phosphorus and it seems prudent to utilize potassium fertilization programs that replace potassium that leaves the farm in harvested product and to use soil testing to monitor the situation.

Table 1. Nutrient content of Salinas Valley crops at harvest (lbs/acre).

1 – higher nitrogen uptake occurs on 5-6 seedlines on 80 inch beds;

2 – these values are currently being evaluated by a research project

Table 2. Soil adequacy levels of potassium for Salinas

Valley cool season crops

Table 3. Analyses of 14 Central Coast soils for available potassium

(ammonium acetate extraction top foot of soil) in 2010

Table 4. Estimate of cations and anions leached during winter

storm events (winter 2009-2010) under three cover crop treatments