Improving fertilizer availability

By James Hoorman, Hoorman Soil Health Services

As fertilizer costs increase, farmers want to either lower their fertilizer costs or find ways to conserve soil nutrients. Cover crops can help do both things.  Legumes and clovers sequester nitrogen (N) and grasses and radishes make phosphorus (P) more available.  Most conventional soil tests measure inorganic soil nutrients but are less reliable accounting for organic or carbon-based plant nutrients. As soil health improves, nutrient availability and nutrient efficiency generally improves due to higher soil microbial activity.

Manure improves soil health and soil organic matter (SOM).  Solid chicken manure is high in N, P, and calcium.  Liquid manures (hog and dairy) can be major sources of nutrients but have a high-water content (dairy, 98% water; hog, 95% water) and with high transportation costs, can be more expensive.  Composting solid manure tends to concentrate available nutrients because as manure decomposes, the volume generally reduces to about a third of the original volume.  Good decomposed compost is an excellent source of soil nutrients, including macro- and micro-nutrients, stimulating soil microbial activity and improved nutrient availability if applied appropriately.

Jim Hoorman, Hoorman Soil Health Services

Most farmers apply nutrients based on the Law of the Minimum, meaning they apply the nutrient that is limiting.  The Law of the Maximum may also apply which means that nutrients that are over applied may tie up other nutrients, especially micro-nutrients which are needed to make proteins and enzymes.  For example, too much nitrogen ties up boron, potassium, and copper.  Too much nitrogen may increase soil magnesium levels, which tends to make our soils hard.  To improve nitrogen efficiency, make sure you have adequate molybdenum which is needed by soil microbes to fix nitrogen. Liming acid soils improves molybdenum and improves nitrogen fixation. So sometimes cutting back a little on fertilizers that tend to be over applied can be beneficial.

For phosphorus (P), the interactions are much more complicated.  High P may decrease calcium, iron, and magnesium but the reverse also occurs; high calcium, iron, and magnesium soil values decrease P availability.  Soils that are compacted and have poor soil structure tend to have higher levels of iron and magnesium and may make P  and calcium less plant available. Often, calcium can be soil limited, so adding lime or gypsum (calcium sulphate) may help improve nutrient availability.   High soil P levels decreases potassium (K), copper, and zinc.  Both liming (when needed) and adding SOM (which acts as a buffer) makes soil nutrients plant available and increases nutrient efficiency. SOM is a storehouse for most plant available micor- and macro-nutrients.

For potassium (K),  too much calcium, N, or P can decrease K plant availability.  Adequate molybdenum may increase plant K absorption and improve N fixation.  Molybdenum and manganese are often foliar applied in the summer.  Manganese deficiency is common in soybeans.  High K values will reduce boron availability.  Potassium (K) availability is reduced when soils are saturated and compacted because the K ion gets tied up in the soil minerology, which is called potassium induction.

Legumes and clovers are cover crops that can fix N.  The highest N fixation comes from cover crops that were planted in the fall.  Hairy vetch (well drained soils), winter peas, alfalfa, white clover, Balansa, and sweet clover may add 120-200 pounds of soil N if allowed to start blooming (1/10 bloom).  Crimson, red, berseem, alsike and sun hemp or cowpeas (both summer annuals) may also add 75-150 pounds of N. For immediate N fixation this spring, try planting lupins and lentils in March for 50-75 pounds of N with good growth. Most legume and clovers need to be planted the previous year and must to be inoculated with the right inoculum at planting to maximize N production.

Several plants, including most grass and brassica cover crops, help scavenge soil nutrients.  While legumes and peas provide N to plants, grasses provide plant available P.

Rye Cover Crop (Photo courtesy of Sustainable Agriculture Research & Education)

Oats, cereal rye, barley, triticale, and annual ryegrass make P plant available.  If your P soil test values are low, buckwheat tends to raise P soil test levels.  Daikon radish are one of the best scavengers of both N and P, but only until they freeze.  If planting radish as a scavenger, plant no more than 2 pounds radish per acre and add to a mixture that includes some winter grass annuals which can recycle those nutrients forward.  Adding manure or compost and planting cover crops improves soil health and is a great way to improve nutrient efficiency and lower your fertilizer bill.

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